The Russian foreign ministry voiced "concern" on Monday about North Korea's nuclear test but was still examining the situation, the RIA-Novosti state news agency reported.
"The information about the North Korean nuclear test evokes concern, but before reaching any final conclusions it must be carefully checked," the ministry's press service was quoted as saying.
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 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.