Japan said such a programme "should never be tolerated", while South Korea voiced "grave concerns", but Washington's special envoy for Pyongyang, Stephen Bosworth, left the door open for engagement.

In parliament, 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Defence Minister Kim Tae-Young raised the possibility of asking the United States to restore tactical nuclear weapons withdrawn from the South in 1991.

His ministry later said he only meant that "all possible options could be reviewed".

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

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)".

Bosworth 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".

The US envoy said he did "not at all rule out the possibility of further engagement with North Korea" and said the six-party process was not dead -- but said there would be no "talking just for the sake of talking".

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

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

Alarm bells rang after a US scientist revealed 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 top military officer, Admiral Michael Mullen, told ABC television the assumption is "that they continue to head in the direction of additional nuclear weapons".

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

As part of a 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 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 asserts its right to be treated as a nuclear state -- something that Washington, Seoul and Tokyo refuse to countenance.

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 that the North, by advertising its plant to Hecker, may be trying among other possible motives 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.

Later Monday Bosworth went on to Japan, whose Foreign Minister Seiji Maehara said the new North Korean plant, if confirmed, represented "a grave situation".

burs-nys/sm/slb

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