North Korea says it has successfully conducted an underground nuclear test, defying international warnings and raising concerns over its nuclear weapons program.
Pyongyang said Monday's test was more powerful than the country's first test two-and-a-half years ago.
South Korea's Yonhap news agency also reports that North Korea test-fired a short-range missile just hours after the nuclear test.
Obama: 'Matter of Grave Concern'
U.S. President Barack Obama said early Monday that the reported tests are a "matter of grave concern" and warrant international action. He said in a statement, "By acting in blatant defiance of the United Nations Security Council, North Korea is directly and recklessly challenging the international community."
Russia's envoy to the United Nations said the U.N. Security Council will hold a special meeting Monday to discuss the issue.
Russia's Defense Ministry said 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. Seismologists from around the world reported a tremor in northeast North Korea a little before 0100 UTC, near where Pyongyang conducted its first test in October, 2006.
Asian Countries React
South Korean President Lee Myung-bak convened an emergency security session immediately after news broke of the nuclear test. The report also shook South Korean financial markets with the main share index falling nearly four percent.
The Japanese government said it has set up a task force at the crisis management center of Prime Minister Taro Aso's office.
Last month, North Korea threatened to restart reprocessing work at its once closed Yongbyon nuclear complex. The move was a retaliation 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.
Pyongyang Defies International Community
In addition to threatening to restart its Yongbyon facility, North Korea has also dropped out of six-party talks aimed at ending its nuclear weapons programs and said it will conduct nuclear and ballistic missile tests.
Washington's top nuclear envoy, Stephen Bosworth, returned from a trip to the region earlier this month that included stops in China, Japan and South Korea.
Bosworth says Washington is ready for direct talks with Pyongyang. North Korea has yet to respond to the offer.
Some information for this report was provided by AFP, AP and Reuters.