Seoul (AsiaNews) – Pyongyang is testing its “biological and chemical weapons” on “humans, specifically mentally or physically handicapped children,” a former officer in the North Korean military who defected to South Korea is quoted as saying on the al-Jazeera website. In the interview the defector said that Pyongyang has estimated stockpile of more than 5,000 tonnes of biochemical weaponry, including mustard gas, nerve agents such as sarin, anthrax and cholera.

Im Chun-yong is a former captain in the North Korean military who fled in 1999 with a number of soldiers. He swam across the half frozen Tumen River into China. During their escape he and his men battled their way out, chased by dozens of members of other commando units. “I myself killed three men,” he told al-Jazeera.

He now lives in South Korea and devotes his time to gathering intelligence about the North's military capabilities

"If you are born mentally or physically deficient,” Im said, “the government says your best contribution to society . . . is as a guinea pig for biological and chemical weapons testing.”

Even after settling in South Korea, he kept the secret because “it was too horrific to recount.” But with Kim's health reportedly failing, and the country appearing increasingly unpredictable, Im felt it was time he spoke out.

The former military captain explained how in the early 1990s, he watched his then commander wrestle with giving up his 12-year-old daughter who was mentally ill. Initially, the commander resisted, but after mounting pressure from his military superiors, he gave in. I “watched as the girl was taken away. She was never seen again,” he said.

One of Im's own men later gave him an eyewitness account of human-testing. Asked to guard a secret facility on an island off North Korea's west coast, Im says the soldier saw a number of people forced into a glass chamber. “Poisonous gas was injected in,” Im said. “He watched doctors time how long it took for them to die.”

In the past North Korean defectors have alleged that the secretive nation has been using political prisoners as experimental test subjects.

Some have detailed how inmates were shipped from various concentration camps to so-called chemical “factories”, including one just a few kilometres north of the capital, Pyongyang, this according to Kim Sang-hun, a retired UN official. "Human experimentation is a widespread practice," he said.

But Im's is the first account of mentally-ill or physically challenged children being used.

He said that he was trained on how to use biochemical weapons against the "enemy".

As the world's attention focuses on the North's nuclear programme, Im is worried that the international community will miss what he believes is the more imminent threat posed by the country's biochemical arsenal.