As North Korea stuck to another go-slow approach, US chief delegate Christopher Hill urged it to close its nuclear reactor, which could produce weapons-grade plutonium, now and give an upfront commitment to disarm.

"I think that the time to stop reprocessing, the time to stop the reactor, is now," Hill told reporters on the second day of the three-day round of talks scheduled to to be reconvened in December.

"Once that is stopped, we look forward to the DPRK (North Korea) making declarations of what it has in the way of nuclear programs."

After a series of plenary and bilateral contacts topped off by a six-party dinner, Hill told reporters that North Korea was "not prepared at this point to tell us when they could shut off the reactor".

The United States, the two Koreas, China, Russia and Japan have held their fifth round of the talks following a September 19 agreement in which the North committed to disarm in return for energy aid and other benefits.

The 5,000-kilowatt experimental graphite-moderated nuclear reactor in Yongbyong has continued operating since the September deal, prompting Hill to declare that the Korean nuclear crisis has "become worse."

Progress in the talks, which began in August 2003, has already stalled over sharp differences about who does what first, with the North insisting the United States supply it with a light-water nuclear reactor before it disarms.

"The three-day session is too soon and too short a time to be working out a complete implementation plan," Hill said.

"But I hope that at our next session, we will be able to make some progress on this and that I can assess how ready the DPRK is to do so."

The current session was planned for at the last round but was squeezed into

a busy diplomatic calendar dominated by an Asia-Pacific summit in the South Korean city of Busan next week.

After a dinner with his North Korean counterpart Kim Gye-gwan on Wednesday, Hill expressed frustration at the pace of progress following the September deal.

"I think the DPRK has a much more deliberately slower process of how to handle denuclearization," he said.

Japan expressed strong support for the US position on Thursday, criticising North Korea for its insistence on the light-water reactor and its stance that the disarmament process should be dragged out through phases.

Japan's chief delegate, Kenichiro Sasae, told his counterparts during Thursday morning's plenary session that the North's position was "not constructive" and urged them to not keep going over old issues.

"It is also not appropriate to consider setting a first stage of measures. This will not help settle negotiations and will make it impossible to agree on and implement action plans," said a statement summarising Sasae's comments.

The nuclear crisis flared in October 2002 after the United States accused North Korea of running a secret uranium-enrichment program.

The North responded by throwing out UN International Atomic Energy Agency weapons inspectors and abandoning the Nuclear Non-Proliferation Treaty.

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