North Korea has launched an aggressive campaign seeking to discredit a UN report on human rights abuses in the country, following news that a prominent defector had recanted parts of his testimony.

Shin Dong-hyuk, subject of the bestselling book "Escape from Camp 14", is a well-known North Korean defector and gave testimony to a UN inquiry that has issued a damning indictment of the North's rights abuses.

On Sunday, Shin admitted in a post on his Facebook page to having changed parts of his story.

"He is a swindler who had appeared with false name and career, and no more than a parasite," Ja Song Nam, North Korea's Ambassador to the United Nations in New York, said in a letter seen by Reuters.

The letter was sent to the UN General Assembly and Secretary-General Ban Ki-moon and the North Koreans asked that it be regarded as an official document.

It repeated North Korea's long-held position that it does not run political prison camps and said Shin was a "criminal who fled after raping a minor girl who was only 13 years old".

North Korea has accused Shin of rape in the past. In an October interview with Reuters, Shin said the accusation of sexual assault was a fabrication that he had heard before.

Shin, who originally said he was born and raised within North Korea's notorious Camp 14 and fled the country in 2005, now says he was transferred to the less brutal Camp 18 in early life, and first escaped to China in 2002.

North Korea's letter said "all the 'resolutions' on the situation of human rights in (North Korea) forcibly adopted by the General Assembly on the basis of such false documents are invalid".

Paragraph 760 of the report, which details the torture of children, cites Shin as saying he was tortured at the age of 14 and had his finger cut off for dropping a sewing machine.

In a statement on his website, "Escape from Camp 14" author Blaine Harden said Shin now says he was tortured at the age of 20, and his finger was damaged by guards after his return from China in 2002.

Michael Kirby, an Australian judge who headed the UN inquiry, said the changes in Shin's account did not affect his commission's findings.

"It's part of the testimony of one witness whose testimony is referred to on one page of a 350-page report that includes the testimony of hundreds of other people, so keep it in proportion," he said.