Russia reaffirmed its opposition to North Korea's nuclear weapons programme in a summit with South Korea on Monday, as Moscow and Seoul concluded a pair of key energy deals.

"Russia continues to support having a nuclear-free peninsula and seeks progress in the ongoing six-party talks," Medvedev said at a Kremlin press conference alongside South Korean President Lee Myung-Bak.

Lee, in Moscow on a three-day visit, said the leaders had a "frank discussion" about Pyongyang's nuclear ambitions, which are at the centre of a worsening international standoff.

Last week the International Atomic Energy Agency (IAEA) said the communist state was about to restart a nuclear reprocessing plant and South Korea warned the six-nation talks on its nuclear disarmament were near breakdown.

Ahead of Monday's summit, the Kremlin said Medvedev and Lee would discuss "the denuclearisation process and the process of fulfilling obligations undertaken by participants in the negotiations."

Russia and South Korea also signed major deals on deliveries of natural gas and joint exploration of the huge west Kamachatka oil field at a Kremlin ceremony.

South Korea's Yonhap news agency valued the deal between Russian gas giant Gazprom and the Korea Gas Corporation at more than 100 billion dollars (68 billion euros), citing the Asian country's government.

"Under the deal signed by Korea Gas Corp. and Gazprom, 10 billion cubic metres of natural gas will be imported every year for 30 years," South Korean Deputy Energy Minister Lee Jae-hoon was quoted as saying.

He told Yonhap the gas would be delivered through North Korea, adding Russia would negotiate with Pyongyang on building the necessary pipelines.

Gazprom however declined to comment immediately on the deal.

Russia also agreed to let South Korean firms take part in joint exploration of a huge offshore oil field near the Kamchatka Peninsula that the Korea National Oil Corp (KNOC) was kicked out of last month.

"We agreed to cooperate in joint exploration of the Kamchatka shelf," Lee said, referring to a field believed to hold up to 3.7 billion barrels of oil.

This summer, the Russian government declined to renew the licence of a consortium led by KNOC and Russian oil firm Rosneft to explore and develop west Kamchatka, in a decision seen as a blow to energy-poor South Korea.

The volume of trade between Russia and South Korea was 15 billion dollars (10.2 billion euros) in 2007 and may reach 20 billion dollars this year, Medvedev said at Monday's press conference.

Lee was also to meet Prime Minister Vladimir Putin, news agencies reported.

Moscow and Seoul also signed agreements to cooperate on high technology and to ease the visa regime between the two countries, and the two presidents said they had discussed building a railroad to link the two countries.

But between Russia and South Korea lies reclusive North Korea, which told the IAEA last week that it would restart nuclear reprocessing at its Yongbyon nuclear facility, used to make weapons-grade material.

Russia -- along with South Korea, the United States, China and Japan -- is one of the five countries engaged in on-again, off-again negotiations with Pyongyang aimed at nuclear disarmament.