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North Korea conducted nuclear tests in 2006 and 2009.

Tuesday’s test was carried out using a “miniaturized and lighter nuclear device with greater explosive force than previously,” the news agency said.

U.S. intelligence officials assessed that the explosion yield was several kilotons.

Mr. Obama said North Korea’s nuclear-weapons and ballistic-missile programs constitute threats to U.S. national security and to international peace. He also spoke on the phone Tuesday with South Korean President Lee Myung-bak.

U.N. Secretary-General Ban Ki-moon, a former South Korean foreign minister, also condemned the tests. Japan, Russia, South Korea and NATO issued statements denouncing the nuclear test.

North Korea was widely expected to test a nuclear device this week because it coincides with the Feb. 16 birthday of Kim Jong-un’s late father and North Korean strongman Kim Jong-il.