A foreign ministry spokesman said the United States is hindering progress in six-party talks by raising "unjust demands."

He rejected suspicions that the communist state has a secret enriched uranium bomb-making programme in addition to its declared plutonium-based operation.

"If the US keeps insisting that what does not exist exists and delays the settlement of the nuclear issue, it would have a serious impact on the disablement of nuclear facilities ...," the spokesman said in a statement carried by the Korean Central News Agency.

"We make it clear we have no uranium enrichment programme, we have not extended any nuclear help to any country. We have never dreamed of such things. There will be never, ever such things," he said.

North Korea in February and October last year signed six-nation agreements under which it would disable its main atomic plants at Yongbyon and declare all its nuclear programmes and materials by the end of 2007.

The US-supervised disablement has been going ahead, but Washington and Pyongyang cannot agree on the declaration.

The North says it submitted the declaration last November. The US says it has not fully accounted for the suspected uranium enrichment programme and for allegations of nuclear proliferation to Syria.

The foreign ministry spokesman said the United States had not followed its part of an October 3 agreement since it had not removed the North from its list of states sponsoring terrorism.

He said North Korea had done all it should have to clear US doubts about uranium enrichment and had gone out of its way to share sensitive military items with US experts.

Pyongyang had also yielded to a US demand that it reaffirm its commitment against nuclear proliferation.

"We have been sincerely carrying out negotiations ... however, the more talks we are engaged in, the greater we get disappointed with the Bush government's attitude," the spokesman said.

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