The remarks from State Department spokesman Philip Crowley reinforced those made earlier by South Korea's foreign minister Kim Sung-Hwan who voiced suspicion that Pyongyang may harbor secret uranium enrichment facilities.

"We're very conscious of the fact that, in the recent revelations to American delegations, what they saw did not come out of thin air. It certainly reflects work being done at at least one other site," Crowley told reporters.

"This remains a significant area of concern," he said, adding he did not want to get into intelligence matters.

The North revealed an apparently operational uranium enrichment plant at its Yongbyon atomic complex to visiting US experts on November 12, shortly before it sparked a security crisis with an artillery attack on a South Korean island.

The New York Times, citing anonymous officials in President Barack Obama's administration, reported Tuesday that North Korea's new nuclear facility is "significantly more advanced" than work done by Iran.

Pyongyang says its new operation is intended to fuel a nuclear power plant, but senior US and other officials fear it could easily be reconfigured to produce weapons-grade uranium to augment the country's plutonium stockpile.

Diplomats are touring the region to discuss a response both to the attack and the potential new nuclear threat.

Chosun Ilbo newspaper, quoting a South Korean intelligence source, said Seoul and Washington believe there may be three or four other locations where the North is conducting uranium enrichment.

"It is a report based on intelligence information and I would just like to say we have been following the issue for some time," Foreign Minister Kim Sung-Hwan told a briefing.

US scientist Siegfried Hecker, one of those to see the Yongbyon plant, said it was most likely designed to make fuel for a civilian reactor and not bombs.

"However, it is highly likely that a parallel covert facility capable of HEU (highly enriched uranium) production exists elsewhere in the country," he wrote in Foreign Affairs magazine.

Kim said: "I can't speak definitely, but I personally think that there is a fair point in Dr Hecker's assumption."

Russia's Foreign Minister Sergei Lavrov, at a meeting Monday with his visiting North Korean counterpart Pak Ui-Chun, expressed "deep concern" about the new uranium capability, Moscow's foreign ministry said.

South Korea is still in shock after the North's November 23 bombardment of Yeonpyeong island near the disputed Yellow Sea border, which killed four people including two civilians.

It was the first attack on a civilian area in the South since the 1950-53 war.

South Korean army chief General Hwang Eui-Don resigned Tuesday over a controversial property investment, in a further blow to the military's morale. It has been widely criticized for a perceived feeble response to the North's attack.

Hwang stepped down following media reports that he had profited unfairly from the property deal, a claim he denies.

But he judged it inappropriate to stay in his post at a time when he must lead reform of the army, a defense ministry spokesman told AFP.

Lavrov urged North Korea to comply with UN Security Council resolutions banning its nuclear activities and called for a resumption of six-party talks aimed at negotiating an end to the North's nuclear programs.

Russia is involved in the stalled talks alongside the two Koreas, China, Japan and the United States.

China, the North's sole major ally, has called for a new meeting of six-party envoys to resolve the latest crisis.

But the United States, Japan and South Korea say a return to negotiations at this point could be seen as rewarding the North's aggression.

They want China, which has failed publicly to condemn its ally for the island attack, to take a tougher line.

Crowley said US Deputy Secretary of State James Steinberg and other US officials who are expected to press China for stronger action on North Korea left for Beijing.

The delegation will also discuss preparations for Chinese President Hu Jintao's upcoming visit to Washington, he added.

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