The United States, Japan and South Korea expressed regret at the delay, which came amid a report that the communist state has slowed down the disabling work it began in November.

It was supposed to have completed the disablement and handed over the declaration by December 31 in return for one million tons of fuel oil or equivalent energy aid, and diplomatic benefits.

The disablement, financed and supervised by the US, had been expected to miss the deadline for technical reasons. But according to a report by Japan's Kyodo News, the North has also told the US it is reducing the shifts of workers carrying out the operation.

A Pyongyang official reportedly warned last week of a slowdown because of a delay in providing the promised energy aid.

The status of work on the declaration was unclear but some analysts predicted it will be delayed for months.

"While the disablement is a technical issue, the declaration is a politically strategic one which requires lots of thought," Kim Sung-Han, an international politics professor at Korea University, told AFP.

"The declaration is seen as a litmus test of whether Pyongyang is really willing to be a nuclear-free state. Given the current stalemate, it must have made no strategic decision yet."

The US State Department Sunday expressed disappointment.

"It is unfortunate that North Korea has not yet met its commitments by providing a complete and correct declaration of its nuclear programmes and slowing down the process of disablement," said spokesman Tom Casey in a statement.

Japan's foreign ministry voiced regret and urged the North "to provide a complete and correct declaration of all its nuclear programmes as quickly as possible."

South Korea urged its neighbour "to faithfully declare all its nuclear programmes at an early date and complete its disabling without any delay," according to a foreign ministry statement.

One problem with the declaration is reaching agreement on how much bomb-making plutonium was produced at the Yongbyon complex in the past. The North used some of this to stage its atomic weapons test in October 2006, lending greater urgency to the six-party process.

According to Japanese media reports, the North has told the US it produced 30 kilograms (66 pounds) of plutonium -- less than the 50 kg estimated by Washington.

A suspected uranium enrichment programme -- the issue which in 2002 wrecked a previous disarmament deal -- is another key hurdle.

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. The North has never publicly admitted any such operation.

Media reports say it has admitted buying special aluminium tubing from Russia, but purportedly for a rocket programme rather than for enriching uranium.

Daniel Pinkston, senior analyst with the International Crisis Group, said it was ambitious to expect total denuclearisation in the coming year.

He expected Pyongyang to wait to assess the policy of incoming South Korean President Lee Myung-Bak, who has promised to take a firmer line with the North.

It could even wait until after the next US administration takes office in January 2009, to ensure any deal with the Bush administration is not overturned, Pinkston told AFP.

"I hope I'm wrong but that's what I would expect," he said.

"From a strict bargaining standpoint, if I were in their shoes I would not play my last card at this point."

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