NEW YORK: World powers on the UN Security Council are united in condemning North Korea's latest nuclear test, with the United States leading calls for tougher sanctions against the pariah state.
Pyongyang has declared its detonation of a "miniaturised" nuclear device at an underground site on Tuesday was a response to US "hostility" and is warning of even stronger action.
North Korea's brazen act has 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 after the test.
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 has been 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, including Australia, to condemn it. Last month North Korea lodged a request to reopen its embassy in Canberra, which had been closed since 2008.
All 15 Council members have backed a statement saying 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 remains 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 US and that any tightening of sanctions would trigger "even stronger second or third rounds of action".
Pyongyang boasted it had tested a "miniaturised" 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 2.57 GMT (1.57 pm AEDT) on 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 neighbour, analysts say.