(Recasts, adds details, U.S. and UN response in paras 3-4, 10-15)
MOSCOW, May 25 (RIA Novosti) - Russia's Defense Ministry confirmed on Monday that North Korea had conducted a nuclear test, and said it would closely monitor the situation.
North Korea announced earlier in the day it had conducted a successful underground nuclear test explosion "as part of the measures to bolster its nuclear deterrent for self-defense."
The ministry's spokesman Col. Alexander Drobyshevsky said: "The test was carried out 80 kilometers (50 miles) to the northwest of the town of Kilchu. The nuclear device had a yield of between 10 and 20 kilotons [of TNT equivalent]."
"We are currently monitoring the situation," he added.
The Russian Foreign Ministry earlier voiced concern over the reclusive communist state's announcement of its second nuclear test. Pyongyang withdrew from the Non-Proliferation Treaty (NPT) in 2003 and conducted its first nuclear test explosion in October 2006.
"The information on North Korea's nuclear test is a cause of concern, but before we draw any final conclusions it should be thoroughly verified," the ministry's press service said in a statement.
Geological survey agencies around the world said they had detected a 4.7-magnitude tremor in the area where the blast is believed to have taken place.
South Korea's Yonhap news agency reported that after Monday's nuclear explosion, Pyongyang test-fired a ground-to-air missile with a range of 80 miles (130 kilometers) from its northeastern Musudan-ri launch site.
The Russian permanent mission to the UN said the Security Council would convene for emergency consultations at 16.00 EDT (20.00 GMT) on Monday and could decide to hold an emergency meeting on the issue.
U.S. President Barack Obama issued a statement saying North Korea's nuclear and missile tests were a "matter of grave concern to all nations" and called for an immediate response from the international community.
"North Korea's attempts to develop nuclear weapons, as well as its ballistic missile program, constitute a threat to international peace and security," the president said.
Obama said Washington would work through the U.N. Security Council and the six-party talks format on North Korea's denuclearization, involving North and South Korea, Russia, Japan, China and the United States, to address the issue.
The six-nation talks were launched in 2003 after Pyongyang withdrew from the NPT Treaty.
Under deals reached in 2007, the North began disabling a nuclear reactor and other facilities at Yongbyon under international supervision, in exchange for economic aid and political incentives.
However, in December last year, a round of six-nation talks ended in deadlock over a U.S. demand that nuclear inspectors be allowed to take samples out of the country from North Korean facilities for further analysis.
The reclusive communist regime had been threatening for several weeks to resume work at its Yongbyon nuclear facility, which produces weapons-grade plutonium, after withdrawing from six-nation talks. The move came in response to international condemnation of an April 5 rocket launch, which Pyongyang said was carrying a communications satellite.
North Korea had also said it would conduct further nuclear tests and rocket launches to ensure its security and defense capability.