PYONGYANG, North Korea - 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 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.
The White House is calling North Korea's latest nuclear test a "highly provocative act" that threatens U.S. security and international peace.
In a statement issued early Tuesday, President Barack Obama promises to "continue to take steps necessary to defend ourselves and our allies." He also urges "swift and credible action by the international community."
Mr. Obama says such efforts "do not make North Korea more secure." Instead, he says, North Korea has "increasingly isolated and impoverished its people through its ill-advised pursuit of weapons of mass destruction."
The test is seen as another big step toward North Korea's goal of building a warhead that can be mounted on a missile. It would also be a bold signal from young leader Kim Jong Un, who took power in December 2011 following the death of his father, Kim Jong Il.
The nuclear test would is also a challenge to the U.N. Security Council, which recently punished Pyongyang for launching a long-range rocket seen as a covert test of ballistic missile technology. In condemning that December rocket launch and imposing more sanctions on Pyongyang, the council had demanded a stop to future launches and ordered North Korea to respect a ban on nuclear activity -- or face "significant action" by the U.N.
Kim Min-seok, a South Korean Defense Ministry spokesman, told reporters that North Korea informed China and the United States of its plans to conduct a nuclear test. It was not clear when Pyongyang told Beijing and Washington.
The U.S. Geological Survey as well as earthquake monitoring stations in South Korea detected an earthquake near a site where North Korea conducted its second nuclear test in 2009, according to the government-funded Korea Institute of Geoscience and Mineral Resources.
South Korean, U.S. and Japanese seismic monitoring agencies put the magnitude of Tuesday's quake between 4.9 and 5.2.
"A natural earthquake normally starts with a smaller tremor followed by a larger one. This quake's strength was the same throughout," according to Yosuke Igarashi, an official at the Japan Meteorological Agency. He declined to elaborate on the length of the quake or other details, saying the agency was studying the data.
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 the December rocket launch.
North Korea's 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.
Associated Press writers Youkyung Lee in Seoul and Yuri Kageyama in Tokyo contributed to this report.