The report said Pyongyang had recently supplied smelted aluminum tubing to US scientists for testing, but it gave no precise details of when or where the tests were carried out, saying US officials wanted to stay silent for fear of exposing key intelligence methods.

US negotiators would now have to ask for an explanation of where the enriched uranium came from, the Post said, adding it was possible such small traces could have come from contamination from other equipment.

Under a six-party deal struck earlier this year, North Korea agreed to disable its plutonium-producing plants this year and issue a full and complete declaration on all its nuclear activities by December 31.

But the process has reportedly hit a key problem -- the North's refusal to address its suspected highly enriched uranium weapons program to the satisfaction of the United States.

US Secretary of State Condoleezza Rice said in an exclusive interview with AFP on Thursday: "It's my hope that the North Koreans will go ahead and file an accurate declaration and we can then move forward."

"I do think the course of disabling has been pretty smooth. It's been cooperative. The North Koreans have taken the steps they said they would take. We've been able to observe them," she added.

The Post said that Washington has repeatedly pointed to Pyongyang's purchase of thousands of aluminum tubes as evidence that the isolated Stalinist state did have a secret enrichment program.

Enriched uranium can be used for fuel, but can also be used in the production of atomic weapons.

The White House refused to comment on the Post report on Friday, but stressed the US administration wanted a full declaration from North Korea on its nuclear program.

"We want a full and accurate declaration from the North Koreans as called for in the six party agreements. We expect them to live up to their commitments," National Security Council spokesman Gordon Johndroe said.

Former UN weapons inspector David Allbright told the Post that it might be possible to match up the uranium traces with information already gleaned about Pakistani material found in Iran, to determine whether there was some accidental contamination.

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