Seoul: North Korea says it has tested a "miniaturized" nuclear device in defiance of U.N. orders to stop building atomic weapons.
Official state media reported on Tuesday that it was conducted in a safe manner.
This third nuclear test could take North Korea closer to its goal of building a nuclear warhead small enough to be mounted on a long-range missile.
Earlier, an earthquake was detected in North Korea just north of a site where the country has conducted nuclear tests.
The South Korean Defense Ministry, which raised its military alert level after the quake, said it was trying to determine whether it was a test. Nuclear blasts can create tremors but they are distinct from those caused by natural earthquakes.
A U.N. nuclear test monitoring organization detected what it called an "unusual seismic event" in North Korea.
The U.S. Geological Survey as well as earthquake monitoring stations in South Korea detected an earthquake just north of a site where North Korea conducted its second nuclear test in 2009, according to the government-funded Korea Institute of Geoscience and Mineral Resources.
"There is a high possibility that North Korea has conducted a nuclear test," said Chi Heoncheol, an earthquake specialist at the institute. Chi said a magnitude 3.9 magnitude earthquake and a magnitude 4.5 earthquake were detected in the North's 2006 and 2009 nuclear tests.
The United States and its allies have been on edge since North Korea said last month it will conduct its third nuclear test to protest toughened sanctions over a December rocket launch that the U.N. called a cover for a banned missile test.
North Korea's powerful politburo vowed to continue firing "powerful long-range rockets," but a statement by state media Tuesday made no mention of a nuclear test.
North Korea's powerful National Defense Commission said Jan. 23 that the United States was its prime target for a nuclear test and long-range rocket launches. North Korea accuses Washington of leading the push to punish Pyongyang for its December rocket launch.
Last October, a spokesman from the commission told state media that the country had built a missile capable of striking the United States, but did not provide further details. A missile featured in an April 2012 military parade appeared to be an intercontinental ballistic missile, but its authenticity has not been verified by foreign experts.