North Korea's nuclear program is shrouded in secrecy, so it's almost impossible to independently verify many of the details of the test. But its claims play into fears among the United States and its allies that Pyongyang is moving closer to the kind of miniaturized nuclear device that it can mount on a long-range missile.
The United States will try to determine if North Korea has tested a uranium weapon for the first time, a senior White House official said. The first two were plutonium bombs.
Despite the North's claims of progress Tuesday, analysts say they believe it is still years away from having the technology to deliver a nuclear warhead on a missile.
"This test isn't going to do that in and of itself, but it is a significant step forward," said Mike Chinoy, a senior fellow at the University of Southern California's U.S.-China Institute.
Condemnation across the globe
After Pyongyang confirmed it had gone ahead with the test in defiance of international pressure, world leaders responded with condemnation.
Among those countries criticizing North Korea were the United States, China, Russia, Japan and South Korea -- the five nations that had been in talks with North Korea for years over its nuclear program.
"This is a highly provocative act" that threatens regional stability, breaches U.N. resolutions and increases the risk of proliferation, Obama said in a statement.
"North Korea's nuclear weapons and ballistic missile programs constitute a threat to U.S. national security and to international peace and security," he said, calling for "further swift and credible action by the international community."
Deputy Defense Secretary Ash Carter told the Senate Armed Services Committee on Tuesday that the test appeared to be timed to coincide with Obama's State of the Union speech Tuesday night.
"They had several other holidays this week that they could have taken advantage of. They tend to like to do this on holidays," Carter said.
South Korea said the test presented "an unforgivable threat to the Korean peninsula's peace and safety."
"North Korea should be responsible for all the serious consequences brought by such an action," said Chun Young-woo, national security adviser to South Korean President Lee Myung-bak, who is near the end of his term in office.
Obama and Lee spoke Tuesday "to consult and coordinate on the response" to the test, the White House said.
Russia's Foreign Ministry issued a statement demanding that North Korea refrain from any nuclear missile program and adhere to U.N. Security Council guidelines.
It condemned the test as an affront to the community of nations: "It's doubly sad that we are talking about the state with which our country has a long history of good neighborliness."
Japanese Prime Minister Shinzo Abe called the testing a "grave threat to the safety of Japan and a serious challenge against international disarmament framework based on the non-nuclear proliferation treaty."
Military jets in Japan were monitoring radioactive fallout, but all appears to be fine, authorities said.
U.N. Secretary-General Ban Ki-moon called the test "a clear and grave violation of the relevant Security Council resolutions."
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