More evidence has emerged suggesting North Korea exported nuclear material to Libya, according to US newspapers.
Libya surrendered its nuclear material a year ago
Scientists testing processed uranium surrendered by Libya to the US last year concluded it came from North Korea, the New York Times reported.
The IAEA, the UN's nuclear watchdog, has said Libya received nuclear material from Pakistan, but has not confirmed a link with North Korea.
The reports coincide with new hopes of talks on the North's nuclear ambitions.
The first suggestion that North Korea exported processed uranium to Libya came in a New York Times report in May.
This said the International Atomic Energy Association (IAEA) had found strong evidence of nuclear links between the two countries as a result of interviews with members of a secret nuclear network set up by Abdul Qadeer Khan, the former head of Pakistan's main nuclear laboratory.
Now, US intelligence officials have told the same paper that American scientists have concluded that the processed uranium is likely to have come from North Korea.
Although they have no sample of North Korean processed uranium to compare the Libyan material with, they have eliminated other possible sources, the paper said.
Scientists also found indications of plutonium produced at North Korea's Yongbyon nuclear reactor in the container carrying the uranium, the Washington Post reported.
IAEA spokesman Mark Gwozdecky told the BBC: "We are certain AQ Khan material went to Libya and Iran, but we've not been able to confirm 100% there were any other countries involved."
The processed uranium that Libya handed over was not weapons-grade, but if enriched further, it could have constituted the core ingredient of a nuclear weapon.
Pressure for talks
The US, South Korea, Japan, China and Russia have all been engaged in trying to persuade the North to abandon its nuclear programme.
Talks with Pyongyang have been stalled since before the US election, but the North has indicated that it is ready to begin negotiations again soon.
Michael Green, a senior director for Asia on the US National Security Council, who is currently touring the region, said in Tokyo that Washington was ready to make a "serious proposal" at the next round of talks.
Before they agree to take part in more talks with the US administration, the North Korean leadership will be closely following President Bush's state of the union address on Wednesday night.
At the same occasion in 2002, Mr Bush branded North Korea part of an "axis of evil" - a move which signalled the start of a more hawkish US policy on the North, and which Pyongyang interpreted as an extremely hostile gesture.