South Korea's foreign minister voiced suspicion on Tuesday that North Korea may have secret uranium enrichment facilities, in addition to the one it disclosed last month.

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.

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.

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 criticised 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 defence 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 programmes.

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. US Deputy Secretary of State James Steinberg is to visit Beijing this week to press for stronger action.

As part of a flurry of regional diplomacy, South Korea's chief nuclear envoy Wi Sung-Lac left Tuesday for talks with his Russian counterpart Alexei Borodavkin on the shelling and the uranium programme.

And New Mexico Governor Bill Richardson will visit North Korea from December 16 to 20, on what is billed as a private trip, to try to calm tensions.

Minister Kim restated calls for the North to show seriousness about disarmament before six-party talks resume and said discussions should cover the uranium enrichment issue.

"We hope China will play a bigger role with a firm voice" to curb the North, he said.