The founder of Pakistan's nuclear weapons program claims that in the late 1990s North Korean officials paid kickbacks to senior Pakistani military figures in exchange for critical weapons technology.

Abdul Qadeer Khan has given a United States-based expert documents that appear to show North Korea's government paid more than $3.5 million to two Pakistani military officials as part of the deal, the expert told The Associated Press Wednesday.

To back up his claim, Khan released what he said was a copy of a North Korean official's 1998 letter to him, written in English, that purports to describe the secret deal.

Khan gave the documents to Simon Henderson of the Washington Institute for Near East Policy, an authority on Pakistan's weapons program. He did so because he has been accused by his government of running a covert nuclear smuggling operation without official knowledge or consent.

"He gave it to me because he regarded it as showing that the story, the perception that he had been a rogue operator was false," Henderson said.

The letter, along with a statement by Khan describing the deal, suggests that at least some top-level Pakistani military officials knew early on about some of Khan's extensive sale of nuclear weapons technology to other countries, including North Korea, Iran and Libya.

U.S.-Pakistan relations could suffer

If that's true, it could deepen the distrust between the United States and Pakistan, which are struggling to set aside their differences and co-operate in the battle against militant extremists in Afghanistan and Pakistan.

The significance of the revelation is in dispute. Henderson said the documents prove Khan's claims that his nuclear arms smuggling network had high-level support from the Pakistani government, but others say the letter bolsters the government's claims it didn't know what Khan was up to.

The Washington Post said it obtained the documents and first reported on them on its website Wednesday after a lengthy effort to authenticate them.

The letter Khan released is dated July 15, 1998, and marked "Secret." It carries the apparent signature of North Korean Workers Party Secretary Jon Byong Ho.

The text says, "Please give the agreed documents, components, etc. to a ... [North Korean Embassy official in Pakistan] to be flown back when our plane returns after delivery of missile components."

The letter never mentions the word nuclear. But Khan's written description of the events surrounding the letter — which was not published by the Post — makes it clear that the Workers Party official was referring to components and plans for Pakistani centrifuges used to enrich uranium.

Highly enriched uranium can be used either to make fuel for nuclear reactors or to form the explosive core of a nuclear weapon.

Attempt to 'shift blame,' military chief says

Jehangir Karamat, a former Pakistani military chief named as the recipient of the $3 million, said the letter was untrue. In an email to the Post from Lahore, Karamat said Khan, as part of his defence against allegations of personal responsibility for illicit nuclear proliferation, had tried "to shift blame on others."

The other official, retired Lt. Gen. Zulfiqar Khan, called the letter "a fabrication."

The Post said the assertions by Khan and the details in the letter could not be independently verified.

But the newspaper quoted one senior U.S. official who said the signature appeared genuine and the contents were "consistent with our knowledge" of the events described. Another intelligence official said the letter contained information known only to a handful of people. Khan has long denied claims that he was working behind his government's back in his covert nuclear technology sales to foreign governments.

"This is a piece of dramatic evidence that Khan did not act as a single rogue agent, but instead was operating at the instruction of others," Cameron said. "I think the main point of this is that Pakistan used this technology to trade for diplomatic advantage."

David Albright, an authority on nuclear proliferation with the Institute for Science and International Security in Washington, disagreed, saying the letter and Khan's narrative are evidence he acted alone.

"It shows that Khan was a rogue agent and that he colluded to provide centrifuge components to North Korea without Pakistani official approval," Albright said.

He said that in Khan's narrative, which has not been released, the scientist claimed he had assured the military that North Korea would not use the centrifuges for its nuclear weapons program, since it already had more advanced technology for that purpose.

Albright said the claim was false, but Pakistani military officials could have found it plausible.