America denies using biological weapons in the Korean War. But North Koreans still claim the US dropped bombs containing disease-carrying insects and food.

n the winter of 1952, Yun Chang Bin recalls, the American bombers flying overhead had become a fact of life. The small detachment of Chinese 'volunteers’ stationed in his village, Hwanjin, 40 miles north-east of Pyongyang, was not a worthwhile target for the US forces supporting the South Korean regime, so rural life went on much as it had done for generations. Oxen ploughed the fields and the local people – those who had not been conscripted into the North Korean military – worked together tending to the rice crops.

But then, one afternoon in early March, Yun was walking home from school when he saw Chinese troops on their hands and knees in the fields. Standing close to the same spot today, he indicates with a sweep of his hand where they were collecting small objects from the frozen ground.

'There were about 30 or 40 of the Chinese volunteer troops spread out across the field,’ Yun, now 72, says. 'They were wearing masks and gloves and some of them had brooms. They were sweeping up something from the ground and others were picking it up and putting it on a fire.’

Curious, Yun approached the soldiers and asked some of the adults who had gathered what was going on. He recalls being told, 'They are catching flies. They came out of the bombs dropped by the American bastards.’

The bombs, Yun explained, had opened – rather than detonated, as conventional weapons would have done – after hitting the ground and released thousands of insects, the like of which Yun had never seen before.

'Some of the flies were bigger than bees and in several colours,’ he says. 'Some were black. Some were larger and yellow. There were huge numbers of them and the volunteers were busy collecting and burning them.’

The insects had been spread over a large area of farmland and many, Yun explains, escaped the mopping-up operation. Disease broke out in the village the following month. 'I remember the adults calling it enbyo, or heat disease,’ Yun says. 'It was terrible. People developed very high fevers, became delirious and complained that their heads hurt. Their muscles ached and they had blisters on their lips. They groaned with the pain and drifted in and out of consciousness. They couldn’t eat anything and just kept asking for cold water.’

With all the local doctors serving at the front, there were no medical facilities in Hwanjin, so there was little anyone could do for those who had been infected, particularly as no one knew what the illness was. Yun says he was later told it was typhoid. 'It killed my father,’ he adds in a matter-of-fact way. 'He lost his appetite, then lost all movement in the lower half of his body, so he was not able to move. He used to ask me to help him to sit up and I had to hold him in my arms.’

Yun Te Ryong died five days after first complaining of feeling unwell, aged 52. According to Yun, in his neighbourhood alone more than 30 people from 50 families died.

The first major conflict of the Cold War, the Korean War started 60 years ago this month, the result of the division of the Korean Peninsula after the Japanese were defeated in 1945. In the north, the Democratic People’s Republic of Korea was supported by the Soviet Union and the People’s Republic of China; the South had the backing of the United States and, when the fighting broke out, troops from a further 15 nations under the flag of the United Nations.

Britain committed 63,000 troops to the fighting, which began as border skirmishes along the 38th parallel but descended into open warfare after North Korean forces invaded on June 25 1950 – although Pyongyang still claims that it was the victim of an invasion. The war went on until a ceasefire was signed on July 27 1953, and a 2.5-mile-wide buffer zone, known as the demilitarised zone, kept the warring sides apart. Historians estimate that the fighting caused the deaths of two million civilians. North Korea and China lost about 600,000 troops; South Korea some 138,000. The US military lost more than 36,500 personnel. British forces suffered 1,078 losses, with 2,674 wounded and 1,060 missing or taken prisoner.

Even before Chinese troops had entered the fray in October 1950, the US Joint Chiefs of Staff had authorised the use of atomic weapons against targets within China if their troops became embroiled in the conflict. And even though the famously aggressive Gen Douglas MacArthur argued for their use, President Truman withheld permission. Historians argue that a nuclear detonation, impossible to conceal from the eyes of the world, would have increased tensions between East and West, but a more insidious form of warfare would have been relatively easy to carry out, and much simpler to dismiss as enemy disinformation.

And there are plenty of men and women who support Yun’s claims that North Korean civilians were attacked with American biological weapons that contained flies, beetles, spiders, crickets and other insects carrying various life-threatening pathogens, from plague baccilus to cholera, anthrax, encephalitis and yellow fever.

Biological warfare can trace its roots back to the ancient world, with spears tipped with poison in the Trojan War, according to Homer’s Iliad. In subsequent conflicts, water sources were poisoned, arrows were dipped in snake venom and pots of live scorpions were hurled at attackers. In the Middle Ages, the corpses of victims of bubonic plague were catapulted over castle walls and there are allegations that British forces operating in North America deliberately exposed native Indians to smallpox through infected blankets.

During the First World War, Germany had an extensive biological warfare programme, although the use of such weapons was prohibited by the Geneva Protocol of 1925. Despite the ban, the potential of fatal diseases as tools of warfare had been identified two decades before the Korean War by the Imperial Japanese Army, which operated a series of clandestine biological, bacteriological and chemical warfare teams throughout China and occupied East Asia from the early 1930s until the end of the Second World War, targeting both civilians and military units. The core research was conducted by one of these teams, Unit 731, in a series of buildings, some of which have been preserved, in the northern Chinese city of Harbin. Then part of Manchukuo, the puppet state in Manchuria that was ruled by the Japanese military, Harbin was well hidden from the prying eyes of the West.

Unit 731 was run by Lt Gen Shiro Ishii, the man said to be responsible for converting Manchuria into one huge biological warfare laboratory under Japanese rule. In September 1931 a section of the Japanese-operated South Manchuria Railway was dynamited; though blamed on Chinese dissidents the attack was engineered by Japanese militarists, an exercise now known as the Mukden Incident. Having annexed the Korean Peninsula more than 30 years earlier and looking to take control of a larger slice of China, Japan now had the excuse to respond with force. The following year, Manchukuo was created. As well as vastly increasing Imperial Japan’s territory, the move provided its military and scientists areas in which they could experiment without scrutiny.

In 1933 Ishii set up the Department of Immunology at the Army Medical College in Tokyo, but was not satisfied with laboratory tests on animals. A year later, he secured funding to transfer his operations to Harbin, where he built the Ping Fan laboratory.

Ishii and hundreds of his colleagues in a dozen similar units roamed across China for 13 years, carrying out experiments planned at Ping Fan. They dropped from aircraft ceramic bombs that contained infected insects; they deliberately discarded food contaminated with pathogens, aware that local people would eat it when they left; mundane objects such as pens and walking sticks were reportedly impregnated with viruses that would then be passed on to anyone who picked them up.

Chinese researchers now believe that two million Chinese people died from Japanese germ weapons used in combat or against civilians. About 6,000 people died in human experiments in Harbin and satellite units throughout the Imperial Japanese Empire; POWs were also reportedly used in some of the experiments – Russian troops captured on the frontier and some downed air crew – although none of them survived their incarceration to tell their tales.

Today, 65 years after Harbin reverted to Chinese control, part of the sprawling two-square-mile Ping Fan laboratory complex remains, despite Japanese efforts to destroy the evidence of what went on. Vast chimneys loom gaunt against the skyline, a guardhouse stands at the main gate and the main office complex has been turned into a museum, with gruesome recreations of what took place at the site.

'It was cruel of the Japanese to do their tests here in China instead of doing them in their own country,’ Jing Chen-min, the director of the Unit 731 Museum, tells me as he gives me a tour. 'They used germs such as plague, cholera and typhus, and used many Chinese people for their experiments, which included vivisections. It was an organised system to conduct experiments on human beings.

'It is easy believe that these stories are science- fiction,’ Jing adds, 'but we have evidence and documents about what the people of Unit 731 did in Harbin. We also have the testimonies of victims, and the stories of former service personnel in Unit 731, who have told us what they did.’

By early 1945 it had become apparent to even the most fervent nationalists that Japan was losing the war. Amid the chaos of the defeat, Ishii’s men tried to cover their tracks and escape back to Japan. Most made it, although in December 1949 the Soviet Union indicted 12 Japanese officers for plotting to use biological warfare and put them on trial in the city of Khabarovsk in the Russian far east. During the six-day trial, each of the defendants confessed to their roles in crimes documented in 18 volumes of evidence. They admitted killing hundreds of Soviet men, women and children in the course of their experiments and field tests. They also linked the Emperor Hirohito to the programme. The show trial was dismissed outside Russia as Soviet propaganda.

After Japan’s surrender, researchers from Washington’s chemical and biological warfare projects were among the first to arrive in Tokyo, and, along with teams of war crimes investigators, were keen to track down senior members of Unit 731. Reports soon appeared in the international press about POWs and civilians being administered infectious agents in medical tests, along with news of Ishii being located by US forces on January 12 1946 (despite rumours that he had been shot dead and his family staging an elaborate 'funeral’ in his home town). Even then he was not arrested but merely confined to his Tokyo home while the investigation against him continued.

Lt Col Arvo Thompson, an officer from Fort Detrick, the US Army’s principal facility for biological warfare research, interrogated Ishii personally. Ishii boasted about inventing a porcelain bomb designed to spread plague – but managed to conceal the full scale of what had emerged at Ping Fan. Col Thomas Morrow, principal assistant to the chief US war crimes prosecutor, was not put off so easily; he visited China to compile a dossier on Ishii’s activities. But shortly afterwards he was reassigned to new duties in Washington and the investigation stalled. By the time the Far East International Military Tribunal had concluded its hearings into wartime atrocities across the region in 1948, and handed down its sentences, biological warfare had been mentioned only once and taken up a mere 10 minutes of the court’s time.

Masataka Mori, a professor of history at Shizuoka University in Japan, who has studied the activities for Unit 731 for many years, believes that Japan’s biological warfare programme was not fully investigated for good reason: Unit 731’s scientists, he says, were granted immunity in return for sharing the fruits of their research with the Americans.

'Before the tribunal, there was discussion among some member countries about putting Unit 731’s leaders on the stand,’ Prof Mori tells me when we meet in Pyongyang, 'but eventually the occupation authorities – led by Gen MacArthur – decided not to try them. The Americans wanted to obtain information about germ warfare from the unit because it was already the early stages of the Cold War.’ The US, in Mori’s opinion – supported by other researchers on this subject – 'wanted to monopolise that information and struck a deal whereby the members of Unit 731 received immunity in exchange for their knowledge’.

While senior members of the unit are said to have returned to civilian life in Japan – in some cases rising to head key pharmaceutical corporations – Ishii, it has been claimed, was retained by the US military as an adviser. Several former members of Unit 731 have told Prof Mori that Ishii and at least two of his top researchers travelled to Korea after the outbreak of the conflict to advise the Americans on strategy, a claim repeated in Japan’s Asahi newspaper in March 1952.

Prof Mori first visited North Korea in 1990 and has returned three times since to carry on his research. He has visited nine sites that reported germ weapon attacks by American forces during the war and interviewed more than 30 survivors. He says there are striking similarities between the diseases and weapons used by the Japanese military in China and those said to have been deployed by the United States against targets in northern Korea. 'The bombs found on the Korean Peninsula were made of metal, while those used in China were ceramic,’ he says, 'but the symptoms reported in North Korea are very similar to those witnessed in China.’

The Victorious Fatherland Liberation War Museum stands on a bend in the Potong River to the north-west of the centre of Pyongyang. Its architecture is severe, typically North Korean, and footsteps on the marble floor echo off the high ceilings. Visitors are welcomed by a huge painting of the beaming Dear Leader, Kim Il-sung, being feted by workers, soldiers, children and women in the traditional Korean chogori dress. One of the dozens of rooms in the museum is set aside for North Korean civilian victims of the conflict, with images of children hideously scarred by chemical weapons – in 1952 the US military was using 70,000 gallons of napalm every day.

A large part of the room is given over to the use of germ warfare. In the centre of one display sits the casing of one of the bombs said to be have been dropped by an American aircraft; it is about 3ft long and with a diameter of about 10in, painted green and with three distinct compartments. This one has no markings that might identify it as American, the guide says, but she claims that others bore the words made in the usa in raised lettering on the casing. An explanatory sign alongside states that 34 species of insects carrying different diseases were identified as having been dropped on North Korea inside bombs like these. On shelves above the bomb are tall glass containers of preserved insects – fleas, ants, spiders. According to the museum guide, there were 804 reported germ bomb attacks across all of North Korea between February and March 1951 alone.

The exhibition also contains an original of the report issued in Peking in 1952 by the International Scientific Commission for the Investigation of the Facts Concerning Bacterial Warfare in Korea and China, set up by the Helsinki-based World Peace Council, an organisation that to this day describes itself as an anti-imperialist, democratic and non-aligned movement for peace. Instigated after the Chinese leader Zhou Enlai sent a telegram on March 8 1952 to the Secretariat to the United Nations detailing claims of 448 germ warfare sorties by the US Air Force, the Commission’s report was compiled by experts from Sweden, France, Italy, Brazil and Russia, as well as Dr Joseph Needham, a respected British authority on Chinese science.

The report contains a series of specific case studies. In one of them, more than 700 voles infected with plague were found in the Kan-Nan district of China in April 1952, including on rooftops and in haystacks, soon after a US aircraft had been seen passing overhead. In another, from the following month, a young woman is said to have found a straw package containing clams on a hillside close to Dai-Dong, North Korea. She took the shells home and cooked them; by the end of the following day, both the woman and her husband were dead from cholera. A search of the hillside, close to a reservoir, turned up several more packages of the infected clams. The Commission stated its belief that the aircraft that had been heard circling before the packages were found had been attempting to drop the clams into the reservoir to infect it. The Commission pointed out that some of the species of insects found during the conflict had never been seen in this part of Asia before, and certainly not in such huge concentrations and at unseasonable times of the year; the illnesses that they brought with them were often equally unheard of.

'In the light of all these and similar facts,’ the report concluded, 'the Commission has no option but to conclude that the American Air Force was employing in Korea methods very similar to, if not exactly identical with, those employed to spread plague by the Japanese during the Second World War.’ It added that the testimony of the hundreds of witnesses interviewed for the report were 'too simple, too concordant and too independent’ to be doubted. Washington dismissed the Commission’s findings.

It is not difficult for the West to dispute anti-American claims emanating from North Korea. The world’s only communist dynasty, headed since its foundation by Kim Il-sung (who died in 1994) and Kim Jong-il, father and son, is, after all, a country widely acknowledged to counterfeit foreign currencies and manufacture synthetic narcotics to sell overseas in return for hard currency that is then spent on developing nuclear weapons and missiles. At present, intelligence estimates that North Korea’s military has between six and eight nuclear weapons, and the Taepodong-1 missile, based on the Scud, can deliver a payload to a target 1,500 miles away. Development of the Taepodong-2 is said to be under way, although a test in 2006 failed. When it is operational, the missile will have a range of about 5,000 miles, which would give it the capacity to hit the mainland United States.

As recently as March, North Korea announced that it would enhance its nuclear weapons capabilities on the grounds that President Barack Obama was trying to bring the regime down. It is precisely this kind of bombast – and the repeated promises to scrap its nuclear technology in return for international aid or recognition, which are inevitably followed by more defiance – that makes it impossible to trust any proclamation emanating from Pyongyang. But evidence would increasingly suggest that the US is not being completely open about what went on in Korea half a century ago.

During the conflict, 36 United States Air Force officers who had been shot down and captured by North Korean or Chinese troops made written and filmed 'confessions’ stating that they had taken part in missions that involved dropping bombs containing germs. (Repatriated in 1953 after the fragile ceasefire – which is still in place today, in lieu of a formal peace agreement – every one of the US air crew retracted their confessions and claimed they were tortured or indoctrinated during their captivity. They were also threatened with court martial if they failed to do so.) One of them, Col Frank H Schwable, broadcast his confession on North Korean radio in February 1953 and claimed that the verbal order to carry out missions to spread cholera, typhus and yellow fever had come all the way from the Joint Chiefs of Staff.

A large printed version of a confession made by Lt Kenneth Enoch, of the 3rd Bomber Wing, is on display in the Victorious Fatherland Liberation War Museum and includes what are purported to be his hand-sketched diagrams of the bombs he said his aircraft delivered. His B-26 was shot down on January 13 1952, during an attack on the North Korean city of Anju, and he is said to have told his captors that he had been ordered to report the dropping of germ bombs as 'duds’ – weapons that failed to detonate.

After being held as a POW for 20 months, he was released when the war came to an end and both sides exchanged prisoners. Now 85 and living in Texas, he recently appeared in an Al Jazeera documentary about the war, and denied that he played a direct role in biological warfare in North Korea, though he hinted that the whole truth about what took place has yet to be revealed by the Pentagon. Asked whether his aircraft had dropped germ bombs, his answer was ambiguous: 'The people who deal in that don’t have to go and fight, and that’s a pretty sweet deal for them,’ he said. The records of Enoch’s mission over North Korea were removed from the official records by US Air Force investigators two months after his capture and one week before he confessed to carrying out germ warfare.

The US military has not changed its stance over allegations that it used biological weapons in Korea. In response to a list of questions submitted to the Pentagon for this article, Major Maureen Schumann issued a brief statement: 'The long-standing US position is that allegations of biological weapons use in the Korean war is “the disinformation campaign that refuses to die”. Our position has not changed. The allegations have proven baseless time and time again.’

Prof Mori shrugs his shoulders. 'The use of germ weapons in war is a breach of the Geneva Convention and I think that is why they are refusing to admit the allegations. The criterion for my judgment is not whether North Korea’s claim is correct or the American claim is right; the criterion is whether the incidents actually happened or not. I went to North Korea and met people who had suffered the effects of germ warfare. They told me their stories, shedding tears and grimacing with anger. They told me what actually happened and I cannot question that.’

Prof Mori says he believes that a new investigation should be carried out into North Korea’s claims, and that it is time the US, China and both North and South Korea opened up their archives and provided unfettered access to their documents.

Sim Dok Hwa, 75, looks over fields outside the hamlet of Chongbori, north of Pyongyang. 'I was one of four boys in my family, but my three brothers died,’ he tells me. 'My grandfather also died in the germ bomb attacks after they landed here.

'I remember it had been snowing and there were patches on the ground where it had collected. There was a big bomb crater, but it was not a bomb like we had seen before. When the bombs fell, they split into two parts when they hit the ground. My grandfather, who was 78 at the time, went to look at them…’

Sim was not allowed to approach the bombs, but recalls that villagers soon found unusual flies with very long legs, clustered together in the furrows of the fields. Chinese troops, wearing face masks and goggles, began collecting the insects and burning them.

In mid-April 1952, about a month after the bombs had landed in the fields, Sim says, disease broke out.

'All the families in the village were fit and strong farmers,’ he says. 'But then, many died. It was a terrible thing for me to lose my family like that. I know that Americans are our enemy, but they should apologise.’