The group led by Sung Kim, director of the Korean Affairs office at the US State Department, left Seoul for Pyongyang by car Tuesday morning, according to the US embassy here.

"They crossed the border by land at around 11:00 am (0200 GMT). They'll return to Seoul on Thursday," US embassy spokesman Max Kwak told AFP.

The visit is a follow-up to recent US-North Korean efforts to break the impasse over the declaration, which is part of the six-nation deal aimed at disabling the communist state's nuclear weapons programmes.

Kim, who arrived at Seoul's Incheon airport late Monday, told reporters he expects "a very detailed and substantive discussion" in the North about issues related to the declaration.

"We of course hope to have significant progress with this visit," he said, adding that his team would also discuss verifying the document.

"Everything is subject to verification. That is what we need to focus on," Kim added.

In last year's landmark deal, the United States, the two Koreas, China, Japan and Russia agreed to grant North Korea energy aid and major diplomatic and security benefits in return for full denuclearisation.

But the talks have been stalled for months by a dispute over the North's nuclear declaration, which they promised to deliver by the end of 2007.

Washington says Pyongyang should not only account for its plutonium programme but also clear up suspicions about an alleged uranium enrichment programme and suspected proliferation -- claims denied by North Korea.

According to numerous reports, the North in a face-saving gesture will merely "acknowledge" US concerns about the two issues in a confidential document to the United States.