A delegation led by Sung Kim, director of the State Department's Korea office, left Seoul by land for Pyongyang for his second trip in less than a month, said a US embassy spokesman in the South Korean capital.

Both sides reported progress after Kim, director of the State Department's Korea office, visited the North last month.

Thursday's visit will be the first by a US official since Washington alleged that North Korea helped Syria construct a reactor which Israeli warplanes destroyed last September.

US Deputy Secretary of State John Negroponte, who was also in Seoul Thursday, said only that Kim's trip is "part of an ongoing process connected with six-party talks."

The North, which staged a nuclear test in October 2006, is disabling its plutonium-producing reactor and other plants under a deal reached last year with the United States, China, Japan, Russia and South Korea.

But disputes over the declaration due last December 31 have blocked the start of the final phase of the process -- the permanent dismantling of the plants and the handover of all material.

The declaration is crucial to verifying that all material, including stockpiled plutonium which could be used for bomb-making, is accounted for.

In return for total denuclearisation, the North would receive energy aid, a lifting of US sanctions, the establishment of diplomatic relations with Washington and a formal peace treaty.

In addition to the declared plutonium operation, Washington said the declaration must clear up suspicions about an alleged secret uranium enrichment programme and about suspected involvement in building the plant in Syria.

The North denies both activities. Under a reported deal, it will merely "acknowledge" US concerns about the two issues in a confidential separate document to Washington.

The main declaration, to be given to talks host China, would detail the plutonium operation.

A South Korean official said the North is expected to present its declaration to the visiting US team but it is unclear whether it is a final version.

"North Korea could show a draft declaration so that the team can review the contents," the unidentified official told Yonhap news agency.

"We have to see whether a final version will be produced or additional discussions will be needed."

Negroponte will also visit Japan and China for talks on regional issues including North Korea. He held breakfast talks with Foreign Minister Yu Myung-Hwan and later met Vice Foreign Minister Kwon Jong-Rak.

They discussed "a whole range of issues of mutual concern," Negroponte told reporters.