North Korea has successfully carried out an underground nuclear test of a new, "miniaturised" device, state media says, in a move widely condemned by the international community.
State-run Korean Central News Agency (KCNA) said the test was conducted in a safe manner and is aimed at coping with "outrageous" US hostility that "violently" undermines the North's peaceful, sovereign rights to launch satellites.
"The high-level nuclear test, unlike in the past, had more explosive power and involved a miniaturised and lighter atomic bomb and was staged safely and perfectly," KCNA said.
The announcement came after seismic activity measuring 4.9 magnitude was registered on Tuesday by the US Geological Survey.
"The danger posed by North Korea's threatening activities warrants further swift and credible action by the international community."
- US President Barack Obama
UN Secretary-General Ban Ki-moon condemned the test, saying it was "deplorable" that Pyongyang had defied international appeals to refrain from such provocative acts.
"The secretary-general condemns the underground nuclear weapon test conducted by [North Korea] today," Ban's spokesman Martin Nesirky said in a statement. "It is a clear and grave violation of the relevant Security Council resolutions."
US President Barack Obama described the test as a "highly provocative act" that "undermines regional stability" and called Pyongyang's nuclear programme a threat to the US, its allies, and to international security.
Several other countries, including the isolated North's closest ally, China, condemned the test. Japan said it was "extremely regrettable" and Russia called it a violation of Pyongyang's international obligations.
The UK said it would begin "urgent consultations with Security Council partners calling for a robust response".
The announcement that the detonated device was "miniaturised" will raise alarm around the globe, with its suggestion that Pyongyang has mastered the technically complex process of producing a warhead small enough to fit on a long-range missile.
South Korean defence ministry spokesman Kim Min-seok, citing seismic data, said the test had an explosive yield of at least six or seven kilotons, significantly more than North Korea's two previous tests in 2006 and 2009.
The South Korean defence ministry raised its military alert level, saying the country was on alert for additional nuclear activity or a missile launch.
The UN Security Council will meet at 14:00 GMT on Tuesday for emergency consultations.
South Korea is the current president of the 15-country Security Council and had been calling for strong action against its neighbour in the event of a nuclear test.
China expressed "firm opposition" to the test, saying its ally had gone ahead with the blast "despite widespread opposition from the international community".
"We strongly urge the DPRK [North Korea] to honour its commitment to denuclearisation, and not to take any actions which might worsen the situation," the foreign ministry said in a statement.
China had made a special effort to try to head off the bomb test, said a UN diplomat who has taken part in recent consultations. China has "special means of communications" with the entourage of North Korean leader Kim Jong-un, the envoy added.
However, Al Jazeera's Marga Ortigas, reporting from Hong Kong, said China's relationship with North Korea might change under the new Chinese leadership due to take over in March.
"It may no longer tolerate what has become a bigger and bigger thorn in its side," she said.
"The new president, Xi Jinping, has made it very clear clear he intends to make relationships between China and the US better. He wants to redefine them and possibly find a way that they can move forward, working with each other."