World powers on the UN Security Council united to condemn North Korea's latest nuclear test Tuesday, and the United States led calls for tougher sanctions against the pariah state.
Pyongyang said its provocative detonation of a nuclear device at an underground site was a response to US "hostility" and threatened still stronger action, defying warnings of United Nations measures.
North Korea's brazen act overshadowed the build-up to US President Barack Obama's annual State of the Union address -- apparently deliberately -- and prompted him to close ranks with his South Korean counterpart Lee Myung-Bak.
"The two leaders condemned this highly provocative violation of North Korea's international obligations. They agreed to work closely together, including at the United Nations Security Council," the White House said.
North Korea's sole international ally is China, which is keen to avoid the chaos that could ensue if the isolated totalitarian regime collapses, but even Beijing was stern in its condemnation of the test.
Foreign Minister Yang Jiechi summoned North Korea's ambassador in Beijing to express "firm opposition" of Pyongyang's action, and China's envoy voted with the other members of the UN Security Council to condemn it.
Russia's foreign ministry said the test showed "contempt for UN Security Council resolutions" and Foreign Minister Sergei Lavrov said: "Such actions that are worth condemnation require an adequate response."
All 15 Council members backed a statement which said the North was in "grave violation" of UN resolutions and highlighted a threat made last month to take "significant action" if Pyongyang staged a new nuclear test.
The council said it would "begin work immediately on appropriate measures."
But North Korea remained defiant.
"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 insisted.
The test presaged what could be yet another round of tension on the Korean Peninsula, where peace has never been formally declared since a war split it between the authoritarian north and pro-Western south in the 1950s.
The North also appeared keen to broaden the conflict, insisting the test was aimed at the United States and that any tightening of sanctions would trigger "even stronger second or third rounds of action."
Pyongyang boasted it had tested a "miniaturized" device, a claim that will fuel concerns it has moved closer to fitting a warhead on a ballistic missile.
International earthquake monitors detected a tremor at 0257 GMT Tuesday 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 used by the Comprehensive Nuclear-Test-Ban Treaty Organization.
South Korea's spy agency predicted its arch-rival might carry out another nuclear test or ballistic missile launch in the coming days or weeks.
Much attention will now be focused on how tough China is prepared to be with its neighbor, 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, an expert at the Sejong Institute in Seoul, said Kim -- who succeeded his late father Kim Jong-Il in December 2011 -- was intent on triggering a crisis that would force the world 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.
Tuesday's explosion had a yield of six to seven kilotons, the South Korean government said, significantly more than Pyongyang's 2006 and 2009 tests but less than that of the first US nuclear strike on Hiroshima in 1945.