The United Nations Security Council has "strongly condemned" North Korea's third nuclear test and vowed to take action.
"The members of the Security Council strongly condemned this test, which is a grave violation of Security Council resolutions," said South Korean Foreign Minister Kim Sung-hwan, whose country is president of the council this month.
Mr Kim said the council would now consider "appropriate measures", as it believes the nuclear test is a threat to international peace and security.
US Ambassador to the UN Susan Rice said that the US and its allies intended to "augment the sanctions regime" already in place due to North Korea's 2006 and 2009 atomic tests.
North Korea confirmed this morning it carried out a third underground nuclear test despite warnings from the international community.
It told the UN disarmament forum in Geneva that the test was an act of self-defence against "US hostility" and threatened further, stronger steps if necessary.
The International Atomic Energy Agency said the blast was twice as big as the 2009 nuclear explosion.
South Korea's military has reportedly been put on higher alert.
UN Secretary-General Ban Ki-moon described the test as "deplorable".
He said the North Korean government had defied international appeals to refrain from such provocative acts.
China summons North Korean ambassador
China said it strongly opposed North Korea's latest nuclear test and urged its ally to abide by its non-nuclear commitment and not take any actions that would worsen the situation on the Korean peninsula.
The North Korean ambassador was summoned to the foreign ministry in Beijing to explain his country's actions.
"It is China's firm stance to realise non-nuclearisation for the Korean peninsula and prevent nuclear proliferation and maintain peace and stability in northeast Asia," China’s Foreign Ministry said.
US President Barack Obama said the test was a "highly provocative act" that hurt regional stability.
He called North Korea's nuclear programme a threat to the US, its allies, and to international security.
Mr Obama said: "The danger posed by North Korea's threatening activities warrants further swift and credible action by the international community.
"The United States will also continue to take steps necessary to defend ourselves and our allies."
Outgoing US Defence Secretary Leon Panetta told Pentagon workers this morning the US is going to have to continue to deal with rogue states like North Korea.
"We just saw what North Korea has done in these last few weeks, a missile test and now a nuclear test," he said.
"They represent a serious threat to the United States of America, and we've got to be prepared to deal with that."
Russian Foreign Minister Sergei Lavrov called on North Korea to abandon its nuclear arms programme and return to six-party talks.
Tánaiste and Minister for Foreign Affairs Eamon Gilmore condemned the test "in the strongest possible terms".
He said it was a major challenge to efforts to advance global nuclear disarmament.
The Irish National Seismic Network (INSN) said it picked up the "explosion-like" event approximately 11 minutes after North Korea detonated the nuclear device.
INSN Director Tom Blake said the explosion from the nuclear test propagated through the ground measuring 4.9 magnitude.