North Korea says it successfully conducted an underground nuclear test that was more powerful than the country's first test some two-and-a-half years ago.
The country's official KCNA news agency reported that Monday's test was "part of measures to bolster [North Korea's] nuclear deterrent for self-defense."
South Korean President Lee Myung-bak convened an emergency security session over the test. The news also shook South Korean financial markets with the main share index falling nearly four percent.
Seismologists from the United States and South Korea said they detected a tremor in the northeastern part of the country a little before 0100 UTC, near where North Korea conducted its first test in October, 2006.
But U.S. State Department officials said they were not able to immediately confirm that a nuclear test occurred.
The Japanese government said it plans to respond to the North Korean test at the United Nations Security Council.
Pyongyang Reacts to International Criticism
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.
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.