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North Korea's third nuclear test triggered an avalanche of condemnation on Tuesday, with the United States leading calls for new UN sanctions.
The unpredictable Pyongyang government said the detonation at an underground site was in response to US "hostility" and warned of even stronger action.
The 15-member UN Security Council "strongly condemned" the blast after emergency talks on the new proliferation crisis.
All the powers backed a statement which said the North was in "grave violation" of council resolutions and highlighted a threat made last month to take "significant action" if it staged a new nuclear test.
"In line with this commitment and the gravity of this violation, the members of the Security Council will begin work immediately on appropriate measures in a Security Council resolution."
UN sanctions were ordered after the North's tests in 2006 and 2009 and while the statement did not mention sanctions, US ambassador Susan Rice applied early pressure to heighten international action.
"To address the persisting danger posed by North Korea's threatening activities, the UN Security Council must and will deliver a swift, credible and strong response," Rice told reporters.
Pyongyang's actions, she warned, "will not be tolerated and they will be met with North Korea's increasing isolation and pressure under United Nations sanctions."
The North said it had tested a "miniaturized" device -- a claim that fuelled concerns it has moved closer to fitting a warhead on a ballistic missile.
"The US and their followers are sadly mistaken if they miscalculate that the DPRK would accept the entirely unreasonable resolutions against it," said Jon Yong Ryong, the first secretary of North Korea's mission in Geneva.
"The DPRK will never be bound to any resolutions," he told the UN Disarmament Forum.
Seismic monitors detected a tremor at 0257 GMT at the Punggye-ri nuclear test site in the northeast of the country. The North's state media triumphantly confirmed a nuclear blast three hours later.
The event measured 5.0 in magnitude, according to monitoring stations of the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO).
North Korea's foreign ministry said the test was only a first step against US "hostility" and that any tightening of sanctions would trigger "even stronger second or third rounds of action."
South Korea's spy agency predicted the arch-rival neighbor might carry out another nuclear test or ballistic missile launch in coming days or weeks.
US President Barack Obama called for "swift" international action against North Korea. Analysts said the timing appeared to be an attention-grabbing provocation just ahead of Obama's State of the Union address on Tuesday.
Russia's Foreign Minister Sergei Lavrov said he expected "adequate" Security Council measures to be taken.
Japan and South Korea stressed the threat to their own national security. UN Secretary General Ban Ki-moon called the nuclear blast "reckless".
China's Foreign Minister Yang Jiechi notably summoned North Korea's ambassador in Beijing to expressed "firm opposition" to the test which threw down a stark security challenge to Obama and China's new leader Xi Jinping.
"North Korea represents a serious threat to the US. We've got to be prepared to deal with that," outgoing US Defense Secretary Leon Panetta said.
Much attention will now be focused on how tough China is prepared to be with its neighbor however, analysts said.
China's economic aid is crucial to the impoverished North, but analysts say Beijing's leverage is limited by its fear of a North Korean collapse.
"Bluntly put, North Korea's new young leader Kim Jong-Un has embarrassed China's leadership with this latest provocation," said Suzanne DiMaggio, vice president of the Asia Society, a New York based policy forum.
Paik Hak-Soon, a North Korea expert at the Sejong Institute in Seoul, said Kim Jong-Un was intent on triggering a crisis that would force the international community to negotiate on his terms.
"The UN is running out of options and probably knows new sanctions would only have a limited impact," Paik said.
The explosion had a yield of six to seven kilotons, significantly more than the 2006 and 2009 tests which both used plutonium, the South Korean government said.
On a technical level, experts are anxious to know if North Korea has switched from plutonium to uranium weaponization.
A statement issued by the North's state KCNA news agency did not specify which fissile material was used.
The explosive yield compared with 15 kilotons in the world's first atomic bomb dropped by the United States on Hiroshima in 1945.