"Monitoring facilities confirmed an underground nuclear explosion, which took place on North Korean territory. This information is being studied and analysed," a ministry official was quoted as saying by Interfax.

The ITAR-TASS news agency also quoted a defence ministry official confirming the North Korean test.

However Russian officials had carried out radiation tests near Russia's short border with North Korea in the Primorye region and found radiation levels were normal, ITAR-TASS reported, citing a spokeswoman for the region's meteorological bureau, Varvara Koridze.

Earlier on Monday, North Korea said it had staged a "successful" underground nuclear weapons test which was more powerful than its previous test of an atomic bomb almost three years ago.

South Korean officials said a tremor was detected around the northeastern town of Kilju, near where the first test was conducted in October 2006.

Russia's foreign ministry expressed "concern" about Monday's test but said it was still studying the situation, in comments carried by RIA-Novosti.

Russia has been a participant in the six-party talks aimed at convincing the reclusive Communist state to scrap its nuclear weapons programme, along with the two Koreas, China, Japan and the United States.

Russia is also a permanent, veto-holding member of the UN Security Council, which last month condemned a North Korean rocket launch, angering Pyongyang.