Sung Kim, director of Korean affairs, would witness the blowing-up of the tower at the Yongbyon complex, a spokesman said Wednesday.

The official, who has headed a US expert team supervising work to make the complex unusable, will cross the inter-Korean land border Thursday.

Yonhap news agency said the communist North is expected to hand over its long-awaited nuclear declaration Thursday through its embassy in Beijing, as part of a six-nation nuclear disarmament pact.

The North is set to topple the tower the next day in the presence of foreign TV crews as a symbol of its commitment to the process.

North Korea last month gave Sung Kim 18,000 pages of operational records from Yongbyon in what Washington described as an important first step for verification.

The promised declaration will be the latest step in six-nation talks which have dragged on since 2003. Some analysts see them going on for years to come as negotiators try to persuade the secretive communist state to give up its weapons.

The declaration -- which is almost six months overdue -- will list all nuclear material, facilities and programmes, US chief negotiator Christopher Hill said this week.

Weapons will be dealt with in the next phase, he said.

The document was promised under a landmark deal struck in February 2007 between the two Koreas, the US, China, Russia, and Japan.

North Korea agreed to disable its nuclear facilities at Yongbyon and declare all its atomic programmes by the end of that year.

The secretive state, which tested a nuclear weapon in October 2006, has been disabling the plutonium-producing plants in return for energy aid. But disputes over the declaration stalled the process.

US suspicions of a secret uranium-enrichment weapons programme and of nuclear proliferation will now reportedly be addressed in a separate document.

The main declaration to China, which chairs the six-party talks, will cover the production and stockpiling of plutonium at the ageing Yongbyon complex.

The North will reportedly declare a 37-kilogram (81-pound) stockpile, less than US estimates.

Hill told reporters its key element is that "the North Koreans, in addition to laying out all their facilities, have to give us a verifiable figure for how much plutonium they have."

In return, President George W. Bush will soon notify Congress of plans to remove North Korea from a list of state sponsors of terrorism, and refrain from penalising the regime under the Trading with the Enemy Act.

"In the next 45 days after that, before those actions go into effect, we would continue to assess the level of North Korean cooperation in helping to verify the accuracy of its declaration," US Secretary of State Condoleezza Rice said this month.

"If that cooperation is insufficient, we will respond accordingly."

Full six-party talks have not been held since last October but a new round is likely to be called soon.

It will focus on verifying the declaration and on preparations for the third and final phase -- the permanent dismantlement of nuclear plants and the handover of all weapons and material.

In return, the North would establish diplomatic relations with Washington and a peace treaty would be drawn up formally ending the Korean war five decades ago.