North Korea's regional neighbors are in crisis mode over what Pyongyang says is a test of a nuclear device -- the second in its history.
South Korea's top security officials are holding crisis conferences in the hours following North Korea's announcement of a nuclear test Monday.
The North's official Korean Central News Agency described the test as "successful" in an official announcement, adding the explosion was more powerful than its first nuclear weapons test in 2006.
Lim Chang-ho, spokesman with the Korea Meteorological Administration in Seoul, says a major seismic event did occur in the North, near the known site of the 2006 test.
He says sensors detected a 4.5 magnitude underground temblor at 9:54 in the morning local time. He says scientists are proceeding on the assumption the quake was artificially created.
Pyongyang Warned of Test
North Korea warned in recent weeks it would conduct a second nuclear test, amid the sharpest decline in North-South Korean relations in 10 years. Pyongyang pulled out of multinational talks to end its nuclear weapons programs last month, and has vowed to produce more weapons material.
Some North Korea experts are surprised about the timing of the test, which comes during a week of national mourning for former South Korean President Roh Moo-hyun. The former president, a passionate supporter of peaceful engagement with the North, committed suicide on Saturday.