North Korean claims to have a working uranium enrichment programme are provocative and disappointing but "not a crisis", the visiting US special envoy for the communist state said Monday.

Stephen Bosworth's comments follow weekend disclosures by a US scientist that he had toured a new uranium enrichment plant in the North -- raising the prospect that Pyongyang is preparing to build a more powerful atomic bomb.

Stanford University professor Siegfried Hecker, in an interview with The New York Times, said he saw "hundreds and hundreds" of centrifuges and was "stunned" by the plant's sophistication.

Hecker said the North Koreans told him 2,000 centrifuges were running, but he could not verify claims that the plant at the Yongbyon complex was already producing low-enriched uranium.

Bosworth termed the announcement disappointing and "another in a series of provocative moves" by the North.

"That being said, this is not a crisis," he said, after talks with South Korean Foreign Minister Kim Sung-Hwan.

"We are not surprised by this, we have been watching and analysing the (North's) aspirations to produce enriched uranium for some time, it goes back several years."

The US envoy said the programme violated a United Nations 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, who will go on to Japan and China, said he does "not at all rule out the possibility of further engagement with North Korea".

But there would be no "talking just for the sake of talking" and the North had to show it was willing to take hard decisions on denuclearisation.

Asked if the six-party talks are dead, the envoy said: "It's still breathing and I still think we have a hope that we are going to be able to resuscitate it."

As part of the six-nation deal, the North in 2008 shut down the ageing gas graphite reactor which had produced enough plutonium for possibly six to eight bombs.

But it abandoned the negotiations in April 2009 and carried out a second nuclear test a month later.

In September last year it announced it had reached the final stage of enriching uranium -- a second way of making nuclear bombs.

The North also told Hecker it was working on an experimental light water reactor at Yongbyon, a plant which would be fuelled by low-enriched uranium.

Hecker said the latest moves might be aimed mainly at eventually generating electricity. "Yet, the military potential of uranium enrichment technology is serious."

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".

South Korea's defence ministry called the development "a serious issue if confirmed".

David Albright, president of the Washington-based Institute for Science and International Security, said that while the plant may be designed to produce low-enriched uranium, it can "be reorganised to make weapon-grade uranium for nuclear weapons".

In emailed comments to AFP, he estimated that 2,000 centrifuges could yield about 26 kg (57 pounds) of such uranium a year, enough for one weapon.

Albright said the North, by advertising its plant to Hecker, may be trying among other motives to seek a "peaceful use exemption" for its enrichment programme or create a new bargaining chip in negotiations.