The United States urged North Korea on Thursday to reverse its decision to ban international inspectors from its main nuclear complex.

"It's a regrettable step but one that is reversible," US State Department spokesman Sean McCormack said.

North Korea had earlier told the UN nuclear monitoring body, the International Atomic Energy Agency, that its inspectors were no longer allowed into the Yongbyon facilities.

It was the latest move by North Korea to block progress on a nuclear disarmament pact agreed to in the six-nation talks that also include China, Japan, Russia, the United States and South Korea.

"The North Koreans over the past month or so have taken a series of steps that we have viewed as not positive, not helpful, and certainly not furthering the process of denuclearizing the Korean Peninsula," McCormack said.

It remained unclear whether a US delegation in North Korea to oversee the dismantling of the nuclear facilities at Yongbyon had also been banned, McCormack said.

North Korea in August reopened Yongbyon and in September informed the IAEA it would resume producing nuclear material in response to US inaction on removing the Stalinist country from the State Department's terrorism blacklist.

Washington insists that North Korea and the other five countries must first agree to a plan for verifying Pyongyang has fully disclosed all of its nuclear activities.

"We, the other five, would need to be able to satisfy ourselves that the declaration the North Korean government provided is, in fact, complete and correct," McCormack said.

US Assistant Secretary of State Christopher Hill, the American envoy for the negotations, returned from North Korea this week after trying to break the stalemate in moving forward on the agreement.

North Korea has pledged to abandon its nuclear programme in exchange for improved diplomatic and economic relations, including shipments of fuel oil and other humanitarian aid.

The IAEA had been monitoring the shutdown of a plutonium reprocessing plant, a nuclear reactor, spent fuel and a fuel fabrication facility.