The UN Security Council has "strongly condemned" North Korea's third nuclear test and vowed to take action against Pyongyang, the president of the Security Council said.
"The members of the Security Council strongly condemned this test, which is a grave violation of Security Council resolutions," South Korean Foreign Minister Kim Sung-hwan, whose country is president of the council this month, told reporters on Tuesday.
"It is a clear and grave violation of the relevant Security Council resolutions"
- Martin Nesirky, UN spokesman
He said the council would now consider "appropriate measures".
North Korea's third nuclear test drew wide condemnation from several countries and organisations, with the US calling it a threat to regional security and Pyongyang's ally China calling for calm.
State media said the country had successfully carried out the underground nuclear test, involving a new "miniaturised", device.
UN Secretary-General Ban Ki-moon said it was "deplorable" that North Korea had defied international appeals to refrain from such provocative acts.
"The secretary-general condemns the underground nuclear weapon test conducted by [North Korea] today," Ban's spokesman Martin Nesirky said in a statement.
"It is a clear and grave violation of the relevant Security Council resolutions."
The Korean Central News Agency said the test was conducted in a safe manner and was aimed at coping with "outrageous" US hostility that "violently" undermines the North's peaceful, sovereign rights to launch satellites.
"The high-level nuclear test, unlike in the past, had more explosive power and involved a miniaturised and lighter atomic bomb and was staged safely and perfectly."
The announcement that the detonated device was "miniaturised" suggests that North Korea has mastered the technically complex process of producing a warhead small enough to fit on a long-range missile.
North Korea says the test was conducted in a 'safe' manner.
"The danger posed by North Korea's threatening activities warrants further swift and credible action by the international community," US President Barack Obama said in a statement.
The magnitude of the "explosion-like event" was roughly twice as large as that of a 2009 nuclear test in North Korea, the Comprehensive Nuclear-Test Ban Treaty Organisation, a group monitoring nuclear tests, said.
NATO said the "irresponsible" nuclear test was a flagrant breach of UN resolutions and a grave threat to international security.
Al Jazeera's Marga Ortigas, reporting from Hong Kong, said China's relationship with North Korea might change under the new Chinese leadership due to take over in March.
"It may no longer tolerate what has become a bigger and bigger thorn in its side," she said.
"The new president, Xi Jinping, has made it very clear clear he intends to make relationships between China and the US better.
Trade with China has enabled the impoverished North to survive for decades since the end of the Korean War.
"At the same all countries should have the right to make use of nuclear activities for peaceful purposes"
- Ramin Mehmanparast,
Iranian foreign ministry spokesman
In neighbouring South Korea, President Lee Myung-bak held emergency meetings in an underground bunker in his office building.
"We have strengthened the readiness of our military by increasing the security alert level to level two," Defence Ministry spokesman Kim Min-seok said.
"We have also operated the South Korea-US joint surveillance platform to closely monitor North Korea's military movements."
Meanwhile, Iran, hit by UN sanctions for its controversial nuclear programme that the West believes is aimed at developing nuclear weapons, called for the destruction of all atomic weapons after North Korea's test.
"We need to come to the point where no country has any nuclear weapons and at the same time all weapons of mass destruction and nuclear arms need to be destroyed," Foreign Ministry spokesman Ramin Mehmanparast said.
"At the same all countries should have the right to make use of nuclear activities for peaceful purposes."