White House spokesman Robert Gibbs refused to divulge intelligence details but said that Pyongyang's "claims, if true, contradict their pledges and commitments they have made repeatedly to the international community."

Gibbs spoke as shock from North Korea's claims to have a working uranium enrichment facility reverberated around the world, and caused particular concern for US allies Japan and South Korea, as the US envoy for North Korea Stephen Bosworth made a regional tour.

Despite the furor over reports first carried by the New York Times, the White House left open the door for negotiations under the six-party framework with Pyongyang, but said any talks must be substantive and serious.

"The administration believes the six-party process can play an important role if and when the North Koreans take that six-party process to move toward denuclearization seriously," Gibbs said.

"We do not wish to talk simply for the sake of talking. The North Koreans have to be serious about living up to their obligations."

North Korea abandoned six-nation disarmament talks, involving Japan, South Korea, Russia and China, and staged a second atomic weapons test in 2009, having amassed enough plutonium for possibly six to eight small bombs.

A US scientist revealed at the weekend that he had toured a modern, new uranium enrichment plant equipped with at least 1,000 centrifuges on November 12 at the North's Yongbyon nuclear complex.

Stanford University professor Siegfried Hecker called the facility "stunning", adding he was told it was already producing low-enriched uranium, although there was no way to confirm this.

"It is possible that Pyongyang's latest moves are directed primarily at eventually generating much-needed electricity," he wrote in a report. "Yet, the military potential of uranium enrichment technology is serious."

Hecker said his guides told him there were in fact 2,000 centrifuges already producing low-enriched uranium to help fuel a nuclear power reactor. They insisted it was for a civilian nuclear electricity programme.

The US State Department said the government was looking into Hecker's claims but was not aware of North Korea having acquired further nuclear capabilities since UN sanctions were last ratcheted up in 2009.

State Department spokesman Philip Crowley called the latest reports a "publicity stunt" that followed a series of provocations by North Korea, which he called a "serial proliferator."

"They have an agenda which... would presume that we will be required to react and potentially to reward this new development. We're not going to do that," he told reporters.

David Albright, a nuclear weapons expert who heads the Institute for Science and International Security (ISIS), said North Korea may be looking to strengthen its bargaining position ahead of new six-party talks.

The United States and its allies have long feared that North Korea is not only intent on building up its own nuclear arsenal but is eager to pass on nuclear expertise and technology to other countries.

In June 2009, the UN Security Council approved new sanctions and widened efforts to inspect international cargo bound for North Korea following Pyongyang's May 25 nuclear test.

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