North Korea insists it has a right to nuclear weapons
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".
Japanese Foreign Ministry: "We'll respond in a responsible manner"
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.
David Loyn, BBC world affairs correspondent
Understanding the motivation behind the actions of this most secretive of powers is never easy, but North Korea does now appear to have moved from a posture of negotiation to confrontation.
Two years ago North Korea agreed to close its main nuclear plant at Yongbyon and reveal its nuclear assets. But it has since refused to allow in inspectors, and has now withdrawn from all negotiations.
Monday's tests come a little over a month after a test of a long-range missile that threatened Asia and could have reached the US.
Neither pressure nor the offer of a large aid package have worked.
When it meets later on Monday, the UN Security Council will issue a strong statement of condemnation. President Obama is threatening action, but there are very few options left short of war.
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.
Oct 2006 - North Korea conducts an underground nuclear test
Feb 2007 - North Korea agrees to close its main nuclear reactor in exchange for fuel aid
June 2007 - North Korea shuts its main Yongbyon reactor
June 2008 - North Korea makes its long-awaited declaration of nuclear assets
Oct 2008 - The US removes North Korea from its list of countries which sponsor terrorism
Dec 2008 - Pyongyang slows work to dismantle its nuclear programme after a US decision to suspend energy aid
Jan 2009 - The North says it is scrapping all military and political deals with the South, accusing it of "hostile intent"
April 2009 - Pyongyang launches a rocket carrying what it says is a communications satellite
25 May 2009 - North Korea conducts a second nuclear test
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