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."
North Korea says it successfully detonated a miniaturized nuclear device at a northeastern test site Tuesday, defying U.N. Security Council orders to shut down atomic activity or face more sanctions and international isolation.
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 U.N. Security Council will hold an emergency meeting Tuesday morning on North Korea's nuclear test. South Korea's U.N. Mission says the closed-door meeting will begin at 9 a.m. (1400 GMT).
South Korea joined the Security Council in January for a 2-year term and currently holds the council's rotating presidency. South Korea's Foreign Minister Kim Sung-hwan is in New York to preside over a council meeting scheduled to begin at 10 a.m. (1500 GMT) on the protection of civilians in armed conflicts.
South Korean Defense Ministry spokesman Kim Min-seok says North Korea informed China and the United States of its plans to conduct a nuclear test.
Kim says that shortly before noon Tuesday an earthquake was detected in North Korea.
The earthquake was detected just north of a site where the country has conducted nuclear tests, providing strong indication that Pyongyang has gone ahead with a highly anticipated third test. There was, however, no confirmation of the test.
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.
The U.N. organization monitoring nuclear tests says it has detected 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.
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 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 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.
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