North Korea says it has staged a “successful” underground nuclear test, prompting international condemnation.
The state says it was more powerful than the previous one in October 2006.
A number of external agencies have confirmed a powerful explosion took place, suspected to be associated with a nuclear test.
US President Barack Obama described the North Korean action as a threat to international peace. Crisis talks were being held in South Korea.
An emergency session of the UN Security Council is being convened by Russia, which currently occupies the council’s rotating presidency.
BBC world affairs correspondent David Loyn says North Korea appears to have moved from a posture of negotiation to confrontation over the nuclear issue.
An official communique read out on North Korean state radio said another round of underground nuclear testing had been “successfully conducted… as part of measures to enhance the Republic’s self-defensive nuclear deterrent in all directions”.
It said the test had been “safely conducted at a new high level in terms of explosive power and control technology”.
The test would “contribute to safeguard the sovereignty of the country and the nation and socialism”, the communique said.
The North gave no details of the test location, but South Korean officials said that a seismic tremor was detected in the north-eastern part around the town of Kilju – the site of North Korea’s first nuclear test.
The US Geological Survey said a 4.7-magnitude quake was detected at 0054 GMT, 10km (six miles) underground.
Geological agencies in both South Korea and the US said the tremor indicated a nuclear explosion.
Russian news agencies quoted the defence ministry as saying said its systems had detected a blast of “between 10 and 20 kilotons” – making it much bigger than the 2006 test, which the US said was less than a kiloton.
Hours after the explosion North Korea test-fired three short-range missiles, South Korea’s Yonhap news agency reported.
Pyongyang has so far not commented on Yonhap’s reports.
The US state department said it was still analysing the available data from the test.
But in a strongly worded statement, President Obama said the North’s pursuit of nuclear weapons and ballistic missiles threatened peace and was in “blatant defiance of the United Nations Security Council”.
“The danger posed by North Korea’s threatening activities warrants action by the international community. We have been and will continue working with our allies and partners in the six-party talks as well as other members of the UN Security Council in the days ahead,” his statement said.
A spokesman for the South Korean President Lee Myung-bak said the test was a “grave challenge” to international non-proliferation efforts, while Japan’s Chief Cabinet Secretary Takeo Kawamura said any nuclear test by the North would be “unacceptable”.
Both have formed crisis management teams, and said they would ask for action from the UN Security Council.
The UK Prime Minister, Gordon Brown, said he condemned the test “in the strongest terms” and said it would “undermine prospects for peace on the Korean peninsula”.
South Korea’s stock market fell 4% on the news, over fears that regional tensions would rise.
The North says it remains under military threat from its historic rival, South Korea, and South Korea’s allies, primarily the US – citing such examples as the annual US-South Korean military exercises undertaken in South Korea.
It says it is entitled to retain a military deterrent.
Last month, Pyongyang pulled out of six-party talks on its nuclear programme, in protest against international condemnation of its test-firing of a rocket on 5 April.
The UN Security Council adopted a statement calling on North Korea to comply with a 2006 resolution banning missile tests.
Pyongyang says its rocket carried a satellite, but several nations viewed it as cover for a missile test.
The six-party talks – involving the US, China, Japan, Russia and the two Koreas – have stalled over the failure of Pyongyang to verify the shutdown of the Yongbyon nuclear plant.
North Korea had agreed to dismantle the facility as part of an aid-for-disarmament deal and, in response, the US removed North Korea from its terrorism blacklist.
But the North now believes it is no longer bound by its previous bilateral agreements with the US and agreements under the six-party talks, reports the BBC’s John Sudworth in Seoul, South Korea.
He says the North, which already faces a stringent sanctions regime, probably thinks it has little to lose.