A State Department spokesman says the delegation will demand to see the complex to clarify its purpose and will not be satisfied with verbal guarantees.

The work came to light in August when South Korea said US spy satellite photos showed thousands of workers burrowing into a mountainside near Yongbyon.

Kim Il Sung, founder of the North Korean state

North Korea has repeatedly denied that building work, at the site of a mothballed nuclear plant, is part of a weapons programme.

The US delegation intends to check what work is taking place and whether North Korea is complying with agreements made in 1994 to freeze nuclear activities.

Under the 1994 agreement, the US agreed to provide North Korea with 500,000 metric tons of fuel oil a year and two light-water nuclear power reactors.

'Specific concerns'

State Department spokesman James Rubin said: "We've made clear to the North Koreans that any attempt ... to pursue a nuclear weapons development programme would be unacceptable.

"We have specific concerns about suspect underground construction, and we will demand assurances that North Korea continues to abide by its commitments.

"Verbal assurances will not suffice. We will press for concrete actions, including our access to clarify the nature of underground construction."

The US delegation, led by envoy Charles Kartman, will be in Pyongyang for three days of talks next month aimed at getting permission to visit the site.

Last week North Korea again denied it was building a secret nuclear complex and said it was ready to open its underground facilities to US inspectors.

The official Korean Central News Agency said the building work was on "civilian underground structures on which any doubt cannot be cast".

It added: "If the US persists in inspecting our underground structures, we can show them to it. However, if they prove to be civilian structures, the US must compensate for its aspersions on North Korea."

But Mr Rubin said he did not expect North Korea to give immediate access to the site.