Earlier in the day North Korea told international nuclear monitors to leave the site of its main nuclear reactor in order to step up moves to restart the plant, the UN's nuclear agency said.

In a statement in Vienna, the UN nuclear watchdog agency said that North Korea had also removed special seals and dismantled the agency's surveillance cameras in the facility at Yongbyon.

"There are no more seals and surveillance equipment in place at the reprocessing facility," Melissa Fleming, the IAEA spokeswoman, told reporters in Vienna.

'No further access'

"[North Korea] further stated that from here on, IAEA inspectors will have no further access to the reprocessing plant.

"[North Korea] also informed IAEA inspectors that they plan to introduce nuclear material to the reprocessing plant in one week's time," she said, citing remarks by a senior agency official in a closed door meeting on Wednesday.

The move is seen as a further sign that North Korea is making good on threats to restart its nuclear programme and apparently walk away from a six-nation aid for disarmament deal agreed last year.

South Korea said it was "very concerned" about the move to restore the facilities.

"The government urges North Korea to resume work on disablement at an early date and actively cooperate for an agreement on the verification protocol," the foreign ministry said in a statement.

'Difficult moments'

On Monday, the chief US envoy to the six-nation talks process, Christopher Hill, played down speculation that North Korea's push to restart the Yongbyon reactor meant the deal was unravelling.

Speaking to reporters in New York, Hill admitted that the process to get Pyongyang to denuclearise "has had its difficult moments in the past and we're certainly experiencing another one now".

But he said Pyongyang's actions were part of "the rough and tumble" of negotiations and dismissed suggestions that talks were dead.

"They've been staking out some very tough negotiating positions ... so yes the negotiating process does continue," Hill said. "Clearly we're seeing a tough line from them in the last month."

North Korea, which tested an atomic weapon in October 2006, began disabling its ageing reactor and other plants 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.

Nuclear analysts have said North Korea would need several months, if not longer, to restart the largely dismantled complex.