North Korea conducted a second nuclear test today, triggering an emergency UN Security Council meeting on the reclusive state's defiant act and drawing global condemnation.
British prime minister Gordon Brown called North Korea "a danger to the world", while US president Barack Obama said Pyongyang's bid to develop nuclear weapons was a threat to international peace and security and the international community would need to respond.
The North's neighbour and long-time benefactor, China, said it was "resolutely opposed" to the test.
Russia, which called the test a threat to regional security, said the blast was about equal in power to the US atom bomb dropped on the Japanese city of Nagasaki in World War Two, or about 20 times larger than the North's one kiloton test in 2006.
But the Vienna-based Comprehensive Nuclear Test Ban Treaty Organization contested that, saying the magnitude of the latest test was "slightly higher than in 2006, measuring 4.52 on the Richter scale, while in 2006 it was 4.1".
Raising tensions further, North Korea test-fired three short-range missiles hours later, Yonhap news agency said.
"North Korea's nuclear and ballistic missile programs pose a grave threat to the peace and security of the world, and I strongly condemn their reckless action," Mr Obama said at the White House. "The United States and the nternational community must take action in response."
Officials in Washington and Beijing said North Korea had warned their governments of the test about an hour before detonation but nearby Japan said it was not given advance notice.
Germany, France, Britain and the EU were among those condemning Pyongyang's act, while UN Secretary-General Ban Ki-moon said he was "deeply worried." The UN Security Council was to hold emergency talks on Monday at 4pm EDT (2000 GMT).
The latest test will confound the international community, which has for years tried a mixture of huge aid pledges and tough economic sanctions to persuade the impoverished North to give up efforts to build a nuclear arsenal.
It is also bound to raise concerns about proliferation, a major worry of the United States which has in the past accused Pyongyang of trying to sell its nuclear know-how to states such as Syria.
Iran, which the West accuses of secretly developing atomic weapons and which conducted a missile test last week, said it had no missile or nuclear cooperation with North Korea.
Nato called for North Korea to refrain from raising tensions further. "These irresponsible actions by Pyongyang pose a serious challenge to peace, security and stability in the Asia-Pacific region," the NATO statement said.
Analysts said the North Korea's test also will force Washington to acknowledge that its leverage over the unpredictable state is at best limited. The United States must hope China will put pressure on Pyongyang, despite its fear of destabilising its neighbour.
The official KCNA news agency said the North had "successfully conducted one more underground nuclear test on May 25th as part of the measures to bolster up its nuclear deterrent for self-defence in every way".
North Korean neighbour China today echoed concerns by other permanent members of the UN Security Council.
"The Chinese side vehemently demands North Korea abides by its denuclearization promises, stop any actions which may worsen the situation and return to the six-party talks process," the foreign ministry said in a statement on its website.
"The Chinese government calls on all sides to calmly and appropriately deal (with the situation)."
Analysts said, however, Beijing was unlikely to back stronger sanctions as part of a new UN Security Council resolution, for fear that North Korea might collapse, releasing a flood of refugees across its border.
The test comes as speculation has mounted that leader Kim Jong-il, his health uncertain after reports of a stroke last year, wants to strengthen an already iron grip on power so he can better secure the succession for one of his three sons.
Reuters