Sung Kim returned to the South Korean capital on Friday after a three-day trip aimed at reviewing work on disabling the North's key nuclear plants at Yongbyon, they said.

"He arrived here on Friday afternoon. He will return home Sunday," a US embassy spokesman said without giving further details.

A six-nation pact calls on the communist country to disable its plutonium-producing sites and declare all atomic programmes and facilities by December 31, in return for energy aid and major diplomatic concessions.

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

Sung Kim's visit comes after the Washington Post said Friday that minute traces of enriched uranium had been found on aluminium tubing from North Korea, alleging this appeared to hint at a secret nuclear programme.

The report said Pyongyang had recently supplied smelted aluminium 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.

US Secretary of State Condoleezza Rice on Friday left the door open to a visit to North Korea if it meets certain conditions, saying the "United States doesn't have permanent enemies."

And in an exclusive interview with AFP on Thursday she said: "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."

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

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 programme.