North Korea has vowed to walk out on international talks to end its nuclear programme, and said it would restore its disabled nuclear reactor.
The unusually strong statement follows criticism by the UN Security Council of its recent rocket launch, which critics say was a long-range missile test.
North Korea says its launch was part of a peaceful space programme, designed to put a satellite into orbit.
China and Russia have appealed for the North to return to negotiations.
China, Pyongyang's closest ally, called for "calm and restraint" from all sides.
A Foreign Ministry statement said that Beijing hoped "all sides will... continue to advance and push forward the six-party talks and the denuclearisation of the Korean peninsula".
Moscow expressed regret at the North's decision, while Japan said it "strongly urges" Pyongyang to return to the negotiating table.
Last week, Japan renewed unilateral economic sanctions against North Korea for another year because of its rocket launch.
The six-party talks, involving North and South Korea, China, Japan, Russia and the US, have seen many setbacks since they began more than five years ago.
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"
5 April 2009 - Pyongyang launches a rocket carrying what it says is a communications satellite
14 April 2009 - After criticism of the launch from the UN Security Council, North Korea vows to walk out of six-party talks
North Korea now says it is walking out for good, after describing the UN action as an "unbearable insult".
The North Korean Foreign Ministry said the UN statement - condemning its rocket launch and tightening existing sanctions - infringed its sovereignty and "severely debases" its people.
The ministry said it would "strengthen its nuclear deterrent for its defence by all means".
The North also said that it would restore its partially disabled Yongbyon nuclear reactor - the fuel source for its 2006 atomic test.
Pyongyang partially dismantled the plant in 2008, as part of an international agreement which guaranteed it aid and diplomatic concessions in exchange for disabling its nuclear facilities.
The BBC's diplomatic correspondent Jonathan Marcus says this latest instalment of the North Korean drama has been seen by many analysts as a predictable attempt by Pyongyang to gain the attention of the new US administration.
How far the North Koreans are really willing to go in unpicking the current deal is not clear, he says.
With growing uncertainty about the internal political dynamics in Pyongyang, and a much tougher sounding leadership in South Korea, it may not be easy to get these talks back on track, our correspondent says.
Pyongyang's defiant response came shortly after the 15-member Security Council unanimously condemned the long-range rocket launch on 5 April.
The council also ordered the UN Sanctions Committee to begin enforcing both financial sanctions and an existing arms embargo imposed after the 2006 tests.
There had been hope that the unified statement could pave the way for a return to the talks, which have stalled over the inability to verify the shutdown of Yongbyon.
North Korea had previously threatened that any criticism of the rocket launch would cause it to walk away from the negotiating table.
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