Hill, an assistant secretary of state, flew from the Osan US air base south of Seoul after spending four days in South Korea for consultations.

His arrival in Pyongyang was reported by China's Xinhua news agency.

The chief US nuclear negotiator will meet his counterpart Kim Kye-Gwan to discuss a declaration -- expected within days -- which should include all the North's atomic programmes and plutonium stockpiles plus any weaponry.

Hill told Xinhua he is "looking forward to seeing the declaration soon."

The list should also account for a suspected separate programme to build bombs using highly enriched uranium (HEU), Hill has said.

"We'll continue to have a discussion on that (HEU), with the understanding that I think we can resolve this matter by the end of the year," Xinhua quoted him as saying in Pyongyang.

The North shocked the world in October 2006 by staging a nuclear test. But some four months later it reached a six-nation accord to disable its plutonium-producing plants and make a complete nuclear declaration in return for major energy aid.

The six parties later set a year-end deadline.

US-supervised disablement work at the Yongbyon nuclear complex, which Hill will visit, is well under way.

The content of the declaration could indicate whether the North is willing to go as far as full denuclearisation, or whether it plans to keep its stockpiled material while shutting down ageing plants.

"Giving up an old facility like Yongbyon is not a huge concession," one diplomat said last week. "They may feel they can sacrifice it."

Negotiators would need to know how much plutonium the North has extracted, and "the hardest part for the North will be to say what they have done in the way of explosive devices."

Hill has said the declaration would be submitted to China, chair of the six-nation talks which began in 2003 and also include the two Koreas, the US, Russia and Japan.

Another six-party round is expected to start in Beijing Thursday, although there has been no official announcement.

In addition to the plutonium programmes, Hill has said the US needs a "complete understanding" of the HEU programme even if it is no longer active.

Speaking to Seoul students Saturday, he reportedly said Washington has "credible evidence" of North Korea purchasing equipment and materials that could be used in a HEU programme.

Hill has also stressed the need for the North to list its proliferation efforts, amid suspicions it may have helped Syria build a nuclear plant.

US accusations that the North was operating a secret HEU programme led to the acrimonious breakdown of a previous nuclear disarmament accord in 2002.

Hill has said he is eager to move on to the final phase next year, under which the North should irreversibly dismantle the plants and surrender its plutonium stockpile as well as any bombs.

If it does, the agreement envisages normalised relations with the United States and Japan, an end to economic sanctions and a peace treaty formally ending the 1950-53 Korean War.

Hill has estimated the plutonium stockpile at 50 kilogrammes (110 pounds), which the Institute for Science and International Security has said could produce about a dozen bombs.

The US is pledged to remove North Korea from its list of state sponsors of terrorism in return for denuclearisation, but it is unclear at what stage it will do that.

Hill last visited North Korea in June in what was the first visit by a top US official since 2002.

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