North Korea says it successfully conducted an underground nuclear test
early Monday that was much larger than its first test in 2006.
South Korean officials say North Korea test-fired three short-range missiles Monday shortly after the nuclear test.
North Korea's official (KCNA) news agency reported the nuclear test was "part of measures to bolster [Pyongyang's] nuclear deterrent for self-defense."
Seismologists from around the world reported a tremor of approximately magnitude 4.5 in northeast North Korea (a little before 0100 GMT), near where Pyongyang conducted its first test in October 2006.
Russian and South Korean military officialssaid the nuclear explosion had a force of up to 20 kilotons, the same size as the bomb the U.S. dropped on Nagasaki, Japan, in 1945.
If correct, the blast would have been 20 times more powerful than the country's first test two-and-a-half years ago.
The top U.S. military official, Admiral Michael Mullen, said it could take a couple of days to verify the test, but he had no reason to doubt North Korea's claims.
An international organization that monitors breaches of the Comprehensive Nuclear-Test-Ban Treaty said the explosion appeared to be only slightly larger than the (4.1 magnitude) blast caused by the 2006 test. The Comprehensive Nuclear-Test-Ban Treaty Organization said it could take several days to definitely confirm that the explosion was caused by a nuclear device.
Reports citing officials who spoke on condition of anonymity said Pyongyang notified the United States and China ahead of the nuclear test.
North Korea threatened last month to conduct additional nuclear and ballistic missile tests. But the timing of Monday's actions caught many analysts by surprise.
Some analysts said North Korea may be trying to gain leverage for negotiations with the international community, although they also said the test could be part of an internal political struggle in Pyongyang. North Korean leader Kim Jong Il, who is believed to have suffered a stroke last year, has not named a successor.
Last month, North Korea also threatened to restart reprocessing work at its once closed Yongbyon nuclear complex. The move was a response to international criticism of its April launch of a rocket it says was fired to put a satellite into space.
The United States and other countries believe the rocket was a test launch for a ballistic missile.