North Korea Monday stood accused of breaking international pledges as its partners in six-party nuclear talks reacted angrily to claims that Pyongyang has built a working uranium enrichment plant.

Japan said such a programme "should never be tolerated", while South Korea voiced "grave concerns" over the weekend revelations by an American scientist who said he had toured the spanking new facility outside Pyongyang.

White House spokesman Robert Gibbs refused to get into intelligence details, but said, "their claims, if true, contradict their pledges and commitments they have made.... to the international community."

South Korean Defence Minister Kim Tae-Young raised the possibility of asking the United States to restore tactical nuclear weapons withdrawn from the South in 1991. But the Pentagon said there were no immediate plans to do so.

"The US and our international partners are consulting on what steps to take in light of this new information. So I'd say it's premature to talk about any specific steps," Pentagon spokesman Colonel Dave Lapan told reporters.

Alarm bells rang after a US scientist revealed he had been shown the new uranium enrichment plant equipped with at least 1,000 centrifuges on November 12 at the North's Yongbyon nuclear complex.

Siegfried Hecker called the facility "stunning", adding he was told it was already producing low-enriched uranium for a civilian nuclear electricity programme, but there was no way to confirm this.

US Defence Secretary Robert Gates said the newly revealed uranium enrichment plant, "assuming that is what it is, obviously gives them the potential to create a number more (nuclear weapons)".

But US envoy Stephen Bosworth, in the region to discuss kickstarting stalled talks with North Korea, said the move was "provocative" but "not a crisis".

"We are not surprised by this," he said in Seoul, arguing that North Korea's attempts to enrich uranium -- in addition to extracting plutonium -- went back "several years".

In 2009 North Korea abandoned the six-nation disarmament talks -- involving the two Koreas, China, Japan, Russia, and the United States -- and staged a second atomic weapons test.

Despite the furore over the revelations, the White House left open the door for negotiations under the six-party framework, stressing though that 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 denuclearisation 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".

Japanese Prime Minister Naoto Kan said a North Korean nuclear weapons programme "should never be tolerated".

His Chief Cabinet Secretary Yoshito Sengoku added the development was "absolutely unacceptable from the point of view of Japan's security and the region's peace and stability".

There was no immediate comment Monday from China, North Korea's closest ally and economic lifeline, which Bosworth was due to visit later in the week.

As part of the six-nation pact, the North in 2008 shut down an ageing gas-graphite reactor that had produced plutonium for its weapons drive.

But after walking out of the six-party talks in April 2009, and staging another atomic test a month later, it announced in September last year that it had reached the final stage of enriching uranium.

In recent months, Pyongyang has indicated conditional willingness to return to dialogue but insists it be treated as a nuclear state.

David Albright, president of the Washington-based Institute for Science and International Security, estimated that 2,000 centrifuges could -- if reconfigured -- yield about 26 kilograms (57 pounds) of weapons-grade uranium a year, enough for one device.

In emailed comments to AFP, he said the North, by revealing its plant, may be trying to seek a "peaceful use exemption" for its enrichment programme or to create a new bargaining chip in negotiations.

Adding to uncertainty about North Korea's intentions, ageing leader Kim Jong-Il is preparing to hand power to his youngest son, Kim Jong-Un.

earlier related report
NKorean nuclear claims contradict Pyongyang pledges: US
Washington (AFP) Nov 22, 2010 - North Korean claims to have built a uranium enrichment plant contradict Pyongyang's obligations, the White House said Monday, as it left the door ajar for "serious" negotiations.

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.