North Korea conducts nuclear test
North Korea has staged a "successful" underground nuclear test, the state-run KCNA agency reports.
The agency says Monday's test was more powerful than an earlier one in October 2006. It did not specify the location of the latest test.
The news confirms earlier reports by officials in South Korea.
South Korea's president is reported to have convened an emergency security meeting and Japan is setting up a task force in the prime minister's office.
Japanese Foreign Ministry spokesman Kazuo Kodama said Tokyo would respond to the test "in a responsible fashion" at the UN, without giving further details.
South Korea has set up a "crisis management" team of officers at general-level.
In Washington, a state department spokesman said the US government said it was checking the report, "but we're not able to confirm at this time".
The European Union has said that the development is "very worrying".
South Korea's stock market fell 4% on the news, over fears that regional tensions would rise.
'Problems resolved'
NUCLEAR CRISIS
# 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
On Monday, the KCNA report said that North Korea's test was "aimed at strengthening its self-defence nuclear deterrent in every way".
"The current nuclear test was safely conducted on a new higher level in terms of its explosive power and technology of its control."
It said the results resolved "scientific and technological problems arising in further increasing the power of nuclear weapons and steadily developing nuclear technology".
The report gave no details of the location of the test.
However, South Korean officials said a seismic tremor was detected in the north-eastern part around the town of Kilju - the site of North Korea's first nuclear test.
Both South Korea's and the US geological agencies said the tremor on Monday morning indicated a nuclear explosion.
The US Geological Survey said the 4.7-magnitude quake was detected at 0054 GMT.
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.
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 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.
