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US President Barack Obama called for "swift" and "credible" action after the underground blast as China expressed its "firm opposition" to the test and Russia also reportedly condemned it as a violation of UN resolutions.
UN Chief Ban Ki-moon said he was "gravely concerned about the negative impact of this deeply destabilizing act" as the UN Security Council prepared to hold an emergency session at 9:00 am in New York (1400 GMT).
The provocative test flew in the face of the UN Security Council's warning last month of a "significant" response to any such move, and came hours before Obama was to deliver his annual State of the Union address to Congress.
Obama complained that the test was a "highly provocative act" that -- following the North's December 12 ballistic missile launch -- undermined regional stability and violated UN Security Council resolutions.
"North Korea's nuclear weapons and ballistic missile programs constitute a threat to US national security and to international peace and security," Obama said in a statement issued shortly before 2:00 am Washington time (0700 GMT).
"The danger posed by North Korea's threatening activities warrants further swift and credible action by the international community," he said, adding that the United States would do whatever was necessary to defend its allies.
The United States, China, Russia and the other major powers face intense pressure to act over North Korea's defiance of sanctions imposed after previous tests in 2006 and 2009.
North Korea said it staged a successful test of a "miniaturized" bomb in a declaration that brought immediate global protests, with the UN nuclear watchdog calling it a "clear violation" of Security Council resolutions.
The UN secretary general also condemned the underground test as "a clear and grave violation" of the resolutions, spokesman Martin Nesirky said.
"It is deplorable that Pyongyang defied the strong and unequivocal call from the international community to refrain from any further provocative measures," Ban added, calling on North Korea to "reverse course."
The United States and South Korea, the council president for February, had both called for firm measures against North Korea if it went ahead with the long-anticipated nuclear test.
China, the isolated North's closest ally, expressed its "firm opposition" to the nuclear test in a foreign ministry statement that urged Pyongyang to "honor its commitment to denuclearisation."
Russia condemned the test as a violation of Security Council resolutions, according to the Interfax news agency, citing a foreign ministry source.
Beijing had made a special effort to try to head off the move, according to a UN diplomat who has taken part in recent consultations.
"The Chinese gave the North Koreans a strong warning against carrying out a test as it became apparent that it was imminent," said the diplomat, speaking on condition of anonymity.
"What the North Koreans have done now is a big challenge to the Chinese," the diplomat said, adding that China, Russia and the United States were likely to quickly agree on a tough response.
The envoy added that sanctions may not be agreed upon on Tuesday, but the "intention" would be made clear.
It took weeks for the Security Council to agree on statements and sanctions after North Korea's nuclear tests in 2006 and 2009, and North Korea's sinking of a South Korean warship in 2010.
China has traditionally shielded its neighbor from international action, warning Western powers against any measures that could cause instability along its frontier.
But the Security Council ordered tougher sanctions against North Korea last month for a rocket launch it staged on December 12, triggering a defiant pledge by Pyongyang to bolster its nuclear deterrent.
The Security Council last month added North Korea's state space agency, a bank, four trading companies and four individuals to its sanctions list.
China agreed to add to the resolution a threat of "significant action" in response to any future North Korean nuclear test.