In a statement, the North Korean foreign ministry said they were "compelled" to take countermeasures by suspending disablement work on August 14 because the US had "postponed" the terror de-listing.

It also said the government is planning to "consider soon a step to restore" the plutonium-producing facility at the Yongbyon nuclear complex but did not say when it would do so.

The statement came shortly after Hu Jintao, the Chinese president, left Seoul after summit talks with his South Korean counterpart, Lee Myung-bak, that included discussions on the North Korean nuclear issue.

Bargaining tactic

But the US sees North Korea's announcement as a bargaining tactic rather than an intention to restart its nuclear facilities.

On Tuesday, the US responded by saying that Pyongyang was "well aware" that it must first agree to a plan to verify the declared nuclear activities before being removed from its blacklist.

"It's obviously something we view with great concern," Robert Wood, a state department spokesman, said.

"It's in violation of the commitments they've made in the six-party framework, and it's clearly a step backward."

Wood said US officials will discuss with China, Japan, South Korea and Russia, the other parties in the six-nation talks, about North Korea's latest move but gave no details.

US nuclear experts are still monitoring the sites at North Korea's Yongbyon nuclear plant.

Reactions

Parties to the nuclear pact want the nuclear disablement work to proceed [EPA]
Pyongyang's move drew reaction from among the leaders of the six countries which are party to the talks.

"We are aware of such moves. We wish to have verification of the North's account of its nuclear activities conducted steadily to lead toward the abandonment of nuclear weapons," Yasuo Fukuda, the Japanese prime minister, said.

"From this perspective, we'd like to discuss the issue with related countries including the United States."

Kim Sook, the South Korean nuclear envoy, said he hoped the North would quickly resume its denuclearisation process.

"At this time, when all nations are putting various efforts into the six-party talks to complete the second stage, I feel regret about the [North's] statement," he said.

In June, North Korea destroyed the cooling tower of the Yongbyon facility in a public demolition to demonstrate its commitment to scrapping its nuclear programme.

It began disabling the plutonium-producing facilities in November, but slowed the work over the dispute on how to verify the nuclear declaration it had handed over to the US.