Chun Yung-Woo said a full declaration of all nuclear weapons programmes was essential to keep a six-nation aid-for-disarmament deal afloat.

"It's time for North Koreans to have the courage to tell the truth," Chun told AFP.

"North Koreans have explained a lot about plutonium. But they seem to be less prepared to tell the truth about the UEP (uranium enrichment programme)."

Chun and his team were briefed last week by Sung Kim, the US State Department's top Korea expert, after Kim's three-day trip to review work on disabling the North's plutonium-producing nuclear plants.

North Korea has agreed to disable its plants at Yongbyon and declare all nuclear programmes by year-end in exchange for a million tonnes of fuel oil or equivalent energy aid.

But the deadline is almost certain to slip because the communist state has not yet explained a suspected highly enriched uranium weapons programme to US satisfaction.

The US says it has good evidence that Pyongyang imported material which could be used for such a programme, even if it is not up and running.

"It is much more important to have a correct and complete declaration rather than to have a timely declaration," Chun said. "We cannot push the deal forward without addressing this issue."

US accusations that the North was operating a secret enriched uranium programme caused a previous nuclear disarmament deal to collapse in 2002.

North Korean officials have never publicly admitted this and repeated the denials when they met Sung Kim last week, according to a source quoted by Yonhap news agency.

The Washington Post said Friday that minute traces of enriched uranium had been found on aluminium tubing handed over by the North. It said the traces could have come from contamination by other equipment.

The North had handed over the tubes to try to prove it had imported them for conventional purposes rather than for a uranium programme.

South Korea's foreign ministry said Monday the disablement work was going smoothly but it was hard to say when it would be finished.

"Having set the year-end as a deadline, the six nations are working flexibly," said spokesman Cho Hee-Yong.

Disablement aims to ensure the plants cannot be restarted for at least a year.

Under the final phase of the pact the plants would be dismantled and all nuclear materials handed over in return for diplomatic relations with the US, a lifting of sanctions and a treaty formally ending the 1950-53 Korean War.

The two Koreas and China will meet in Pyongyang for three days from Tuesday to discuss providing energy aid under the six-nation deal. Talks will focus on how to provide non-fuel aid, such as equipment to patch up power plants.

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