U.S. President Barack Obama called a nuclear test on Tuesday by North Korea a "highly provocative act" and said Pyongyang's "threatening activities" warranted action by the international community.
North Korea's third nuclear test came less than 24 hours before Obama's annual State of the Union address, a televised speech viewed by millions of Americans that lays out his priorities for the coming year.
U.S. intelligence agencies were analyzing the event and had found that North Korea probably conducted an underground nuclear explosion with a yield of "approximately several kilotons," the Office of the Director of National Intelligence said in a statement that called it a "declared nuclear test."
"North Korea announced today that it conducted a third nuclear test. This is a highly provocative act that, following its December 12 ballistic missile launch, undermines regional stability," Obama said, adding that the test violated United Nations and other international agreements.
"The danger posed by North Korea's threatening activities warrants further swift and credible action by the international community. The United States will also continue to take steps necessary to defend ourselves and our allies," Obama said.
North Korea is banned under U.N. Security Council resolutions from developing nuclear and missile technology. The tests drew condemnation from around the world.
"North Korea's nuclear weapons and ballistic missile programs constitute a threat to U.S. national security and to international peace and security," 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."
The U.N. Security Council will hold an emergency meeting on North Korea's apparent nuclear test at 9 a.m. EST (1400 GMT), when the United States, South Korea and others could begin the lengthy process of pressing for more sanctions on Pyongyang.
"We will strengthen close coordination with allies and partners and work with our six-party partners, the United Nations Security Council, and other UN member states to pursue firm action," Obama said. He did not specify what action might be taken.
The six-party talks between China, the United States, North and South Korea, Japan, and Russia are aimed at curbing North Korea's nuclear weapons program.
"These provocations do not make North Korea more secure," Obama said. "Far from achieving its stated goal of becoming a strong and prosperous nation, North Korea has instead increasingly isolated and impoverished its people through its ill-advised pursuit of weapons of mass destruction and their means of delivery," he said.
The magnitude of the test was roughly twice as large as that of 2009, said Lassina Zerbo, director of the international data center division of the Vienna-based Comprehensive Nuclear-Test Ban Treaty Organization. The U.S. Geological Survey said that a seismic event measuring 5.1 magnitude had occurred.