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The North Atlantic Council, NATO’s political governing body, condemned North Korea’s test in the “strongest terms” and said it was a “flagrant violation of relevant United Nations Security Council resolutions.”

“This irresponsible act, along with the December missile launch, poses a grave threat to international peace, security and stability. North Korea’s pursuit of weapons of mass destruction represents continued defiance of the UN Security Council and the broader international community,” the council said.

Earlier on Tuesday, the United States, China, Russia and South Korea detected seismic activity in the northern part of North Korea. The Korean Peninsula is not prone to earthquakes, and the tremors led to speculation that Pyongyang had made good on its promise to conduct a nuclear test.

The U.S. Geological Survey said it had detected a magnitude-4.9 quake in North Korea. The tremor was recorded at 11:57 a.m. local time (9:57 p.m. EST Monday).

North Korea’s Punggye-ri nuclear test site nuclear test site is located in the northeastern part of the country. Recent satellite photographs of the site showed that it was in a state of readiness and a test could be conducted within weeks of an order being given, according to an analysis by 38 North, a program of the Johns Hopkins University School of Advanced International Studies’ U.S.-Korea Institute.

North Korea’s nuclear weapons and ballistic missile programs constitute a threat to U.S. national security and to international peace and security,” Mr. Obama said. “The United States remains vigilant in the face of North Korean provocations and steadfast in our defense commitments to allies in the region.”

North Korea last month threatened to conduct a third nuclear test and launch more long-range rockets in response to a U.N. resolution that reprimanded Pyongyang for launching a rocket in December and imposed new sanctions.

The test was widely expected to be carried out this week as it coincides with the Feb. 16 birthday of Kim Jong-il, the former North Korean strongman and father of current leader Kim Jong-un.