North Korea announced on Tuesday that it had conducted its most powerful nuclear test to date, in a striking act of defiance that drew condemnations from around the globe.
The nuclear test Tuesday, which follows previous detonations by the North in 2006 and 2009, had greater explosive force and utilized a smaller, lighter device, the North's state-run Korean Central News Agency (KCNA) reported.
"The test was carried out as part of practical measures of counteraction to defend the country's security and sovereignty in the face of the ferocious hostile act of the U.S.," the report said.
North Korea's claim is of particular concern as it suggests that the isolated country it is a step closer to fitting a nuclear warhead onto a ballistic missile, following its successful launch of a long-range rocket in December.
The confirmation from state news agency KCNA came nearly three hours after seismic monitors detected an unusual tremor at 0257 GMT in the area of the country's Punggye-ri nuclear test site, close to the Chinese border.
Analysts said the timing appeared to be an attention-grabbing calculation from a state well-versed in provocative acts, coming just ahead of President Barack Obama's State of the Union address at the start of his second term, AFP reported.
Obama denounced the test and called for a "swift" and "credible" international response, as the Security Council prepared to meet to discuss the volatile situation.
Condemnation came from others including Russia, while Japan saw a "grave threat" to its own security.
China, whose trade and aid are a life-support to impoverished North Korea, expressed "firm opposition" to the nuclear test and noted that it came "despite widespread opposition from the international community".
"We strongly urge the DPRK (North Korea) to honor its commitment to denuclearisation, and not to take any actions which might worsen the situation," the foreign ministry said in a statement.
The United Nations Security Council was expected to debate new measures during its meeting Tuesday in New York.
While the United States and its allies will urge China to ramp up existing sanctions on its ally, China’s leverage is limited by its fear of a North Korean collapse and the prospect of a reunified, US-allied Korea directly on its border, analysts say.
The foreign ministry statement called "on all parties to respond calmly" and to rely on dialogue in a moribund "six-party" process involving the two Koreas, China, the United States, Russia and Japan.
It was the North's first nuclear test since its new leader, Kim Jong-Un took over from his father, Kim Jong-Il. Security analysts said it sent a bold message of intent following a successful long-range rocket launch in December.
"The launch and the test are empirical evidence that Kim and his regime have no intention of negotiating away the North's nuclear program any time soon," said Paul Carroll, program director at the US-based Ploughshares Fund, according to AFP.
Meanwhile, experts are speculating as to whether North Korea has switched from plutonium to a new and self-sustaining nuclear program using uranium.
The KCNA statement did not specify what fissile material was used, but noted that the test's success had provided the North with a "diversified" nuclear deterrent.
The North has substantial deposits of uranium ore and it is much easier to secretly enrich uranium, which can be done in centrifuges, rather than the nuclear reactor required for plutonium enrichment.
South Korean defense ministry spokesman Kim Min-Seok told reporters that Tuesday's explosion had a yield of six to seven kilotons, significantly more than the 2006 and 2009 tests, which both used plutonium.
The explosive yield compared with 15 kilotons in the world's first atomic bomb dropped by the United States on the Japanese city of Hiroshima in 1945.
North Korea's first test yielded less than one kiloton and was widely seen as a dud while the second test reportedly yielded between two and six kilotons.
The third test throws down a stark security and diplomatic challenge to Obama as well as to new Chinese leader Xi Jinping.
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, as quoted by AFP.
"The only real option for curbing further provocation is starting a dialogue with the North, but that will be very difficult given the domestic political pressure on leaders in the US, South Korea and Japan," Paik added.