China was under pressure Tuesday to curb the atomic ambitions of its ally North Korea, after Pyongyang's latest nuclear claims sparked international alarm.

US envoy Stephen Bosworth was heading to Beijing to discuss the North's disclosure to visiting US experts of an apparently operational uranium enrichment programme -- a second potential way of building a nuclear bomb.

Leading South Korean newspapers called on China to use its influence with its impoverished neighbour, while other papers said the disclosure highlighted the failure of Seoul and Washington's tough policy towards the North.

Bosworth, US special representative for North Korea policy, told reporters in Tokyo that Washington was reaching out to other partners in six-nation talks on denuclearising North Korea -- China, Russia, Japan and South Korea.

He said China and other parties were committed to a September 2005 denuclearisation accord, "but we are very concerned as to the sincerity of the (North's) approach to this".

The White House said the uranium enrichment claims contradict Pyongyang's past pledges but left the door ajar for "serious" negotiations.

"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," spokesman Robert Gibbs said Monday.

"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."

Bosworth, who also visited Seoul on Monday, said Washington does not contemplate resuming negotiations "while active programmes are underway or while there is a possibility that North Koreans will test another nuclear device or test a missile".

The North shut down its ageing gas graphite reactor in 2008 under a six-party deal, after stockpiling enough weapons-grade plutonium for possibly six to eight small bombs.

But it abandoned the forum in April 2009, a month before its second nuclear test, and announced in September last year it had reached the final stage of enriching uranium.

The North, showing off its centrifuges to the US experts this month, said the operation would fuel a civilian electricity-generating project.

But scientists said they could be configured to create weapons-grade uranium, and top US defence officials said that is the real intention.

"It is possible that Pyongyang's latest moves are directed primarily at eventually generating much-needed electricity," Stanford University professor Siegfried Hecker wrote in a report after visiting the new facility at the North's Yongbyon complex.

"Yet the military potential of uranium enrichment technology is serious."

The US special envoy to the six-party talks, Sung Kim, called on China to take a more active stance on North Korea.

"In the six-party context, we want them (the Chinese) to be pro-active," and not just mediators, he said in a talk at a Washington think-tank.

Kim said North Korea "is clearly destabilising to the region and China knows that," adding, "We do share a common goal."

Kim's South Korean counterpart Wi Sung-Lac visited Beijing on Monday and expressed "grave concern" at the latest nuclear revelations.

Wi and his Chinese counterpart Wu Dawei discussed possible responses and pledged to try harder to restart the six-party talks, Seoul's foreign ministry said.

South Korea's Dong-a Ilbo newspaper urged Beijing to press Pyongyang.

"It would be such a cowardly act if China, as the chair of the six-party talks, does not actively join international efforts to denuclearise the Korean peninsula," it said.

The best-selling Chosun Ilbo said China should realise "that a failure to act in time would pose a grave crisis to northeast Asia".

The Korea Times said the North was sending a strong message to the United States -- that the two sides must establish diplomatic ties and sign a non-aggression pact.

If Seoul and Washington maintain the current policy of no talks before denuclearisation, the paper said, this "will lead to the mushrooming of 30-40 nuclear bombs in the North and a possible nuclear proliferation beyond North Korea".