South Korea's army chief resigned Tuesday over a controversial property investment, at a time of high tensions with North Korea following its deadly artillery attack on a border island last month.

The departure of General Hwang Eui-Don is a further blow to the South's military, widely criticised for a perceived feeble response to the North's bombardment, which triggered international alarm.

Pyongyang's newly disclosed uranium enrichment programme has also sparked fears it could produce more material to make nuclear bombs.

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 capability, Moscow's foreign ministry said.

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.

The bombardment of Yeonpyeong island, near the disputed Yellow Sea border, killed four people including two civilians. It was the first attack on a civilian area in the South since the 1950-53 war.

The South fired 80 artillery rounds at the North's artillery batteries in response but did not call in air strikes. The military has said it will use air power next time.

The attack was launched less than two weeks after the North disclosed an apparently operational uranium enrichment plant at its Yongbyon atomic complex to visiting US experts.

It said the operation was intended to fuel a nuclear power plant. But senior US and other officials fear it could be reconfigured to produce weapons-grade uranium to augment the North's plutonium stockpile.

Chosun Ilbo newspaper, quoting an unidentified South Korean intelligence official, said Seoul and Washington believe there may be three or four other locations where uranium enrichment is being conducted.

US scientist Siegfried Hecker, one of the visitors 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.

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 one of the six countries 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.

The North repeated assertions that its new programme is peaceful.

"The business of peacefully developing nuclear energy and using it is happening in our country, in line with the international trend," said ruling communist party newspaper Rodong Sinmun Tuesday.

"Peaceful nuclear activity is a sovereign right of all nations."