On Monday Condoleezza Rice, the US secretary of state, told reporters on the sidelines of the UN General Assembly in New York that "the North Koreans invited Chris Hill to come, so we hope that there is some effort to address the verification protocol because that is what we need".

Last week it barred UN inspectors from the site of its main nuclear plant at Yongbyon, in an apparent move to restart work at the reactor.

Robert Wood, a spokesman for Rice, said Hill was going to Pyongyang "to get a sense on the ground" and "talk with North Korean officials" about why the country had begun reversing its disarmament process.

"We're very concerned about some of the reversal of disablement activities that the North has been engaged in," Wood said. "We want to get the process back on track."

Hill is also set to visit Beijing and Tokyo to discuss "ways to work with our allies to bring North Korea into compliance with its obligations", Wood told reporters.

Sticking point

Hill, a US assistant secretary for East Asian and Pacific affairs is expected to try to encourage North Korea to agree to measures allowing experts to verify an accounting of its nuclear programmes submitted in June.

Last week he conceded that North Korea was taking a "tough line" but added that its actions were part of "the rough and tumble" of negotiations and dismissed suggestions that disarmament talks were dead.

North Korea, which tested an atomic weapon in October 2006, began disabling its ageing reactor and other facilities at Yongbyon last November under a pact with South Korea, China, Japan, Russia and the United States.

But it announced last month that it had halted work in protest against Washington's refusal to drop it from its blacklist of alleged state sponsors of terrorism, as promised under the deal.

Washington says the North must first accept strict outside verification of the nuclear inventory that Pyongyang handed over in June, with Rice saying in New York that "we need to move forward on the verification protocol".

"I think everybody is in agreement ... among the five parties, and so we will look to see what they [the North Koreans] have to say."

The North says verification is not part of this stage of the agreement and accuses the US of violating its dignity by seeking "house searches" as in Iraq.