North Korea's claimed uranium enrichment programme "should never be tolerated" and was "absolutely unacceptable", Japan said Monday as Pyongyang's nuclear scheme sparked anger in the region.

"Nuclear weapons development by North Korea should never be tolerated," prime minister Naoto Kan told reporters. "Japan will maintain the position and cooperate firmly with the United States and other nations."

His comments followed disclosures by a US scientist that he had toured a modern uranium enrichment plant in the communist state, raising the prospect that Pyongyang is preparing to build a more powerful atomic bomb.

Japan's top spokesman, Chief Cabinet Secretary Yoshito Sengoku, told reporters: "North Korea's nuclear development is absolutely unacceptable from the point of view of Japan's security and the region's peace and stability."

US scientist Siegfried Hecker revealed at the weekend that he had visited a new equipped with at least 1,000 centrifuges on November 12 at the Yongbyon nuclear complex.

Hecker, a Stanford University professor, called the facility "stunning", adding he was told it was already producing low-grade uranium, although there was no way to confirm if the plant was fully operational.

Japan's Foreign Minister Seiji Maehara later said that "if the explanation given by North Korea is true, it is a grave situation. It is feared it would violate the series of UN resolutions," he said according to Jiji Press.

Maehara was speaking at a meeting with the US special envoy for North Korea, Stephen Bosworth, who was on a regional tour.

Bosworth, on an earlier stop in South Korea, said Pyongyang's claims were provocative and disappointing but "not a crisis".

The US envoy was next headed for China as he visited nations that are members of the moribund six-party talks aimed at abolishing North Korea's nuclear programme.

Bosworth said in Seoul that the programme violated a UN resolution and a September 2005 six-nation agreement, under which the North agreed to scrap its nuclear programmes in return for aid, diplomatic and security benefits.

However, Bosworth also said he does "not at all rule out the possibility of further engagement with North Korea".

North Korea, which has carried out two nuclear tests, withdrew from the denuclearisation talks in 2009.

The regime also announced last year it was restarting its Yongbyon complex, outside the capital Pyongyang, despite UN condemnation and sanctions.

Hecker, who is reported to have briefed the White House, said he had been astonished by what he had seen.

"Instead of seeing a few small cascades of centrifuges, which I believed to exist in North Korea, we saw a modern, clean centrifuge plant of more than 1,000 centrifuges all neatly aligned and plumbed below us," he wrote.

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