The North said the test had "greater explosive force" than the 2006 and 2009 tests. Its KCNA news agency said it had used a "miniaturized" and lighter nuclear device, indicating that it had again used plutonium which is more suitable for use as a missile warhead.

North Korean leader Kim Jong-un, the third of his line to rule the country, has presided over two long-range rocket launches and a nuclear test during his first a year in power, pursuing policies that have propelled his impoverished and malnourished country closer to becoming a nuclear weapons power.

U.S. President Barack Obama labeled the test a "highly provocative act" that hurt regional stability and he and other states that are members of U.N. Security Council pressed for new sanctions against the isolated country.

"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," Obama said in a statement.

The Security Council will meet on Tuesday to discuss its reaction to the test, although North Korea is already one of the most heavily sanctioned states in the world and has few external economic links that can be targeted.

Japanese Prime Minister Shinzo Abe said the test was a "grave threat" that could not be tolerated. UN Secretary-General Ban Ki-moon said the test was a "clear and grave violation" of UN Security Council resolutions.

China, which has shown signs of increasing exasperation with the recent bellicose tone of its neighbor, summoned the North Korean ambassador and protested sternly, the Foreign Ministry said.

Foreign Minister Yang Jiechi said China was "strongly dissatisfied and resolutely opposed" to the test and urged North Korea to "stop any rhetoric or acts that could worsen situations and return to the right course of dialogue and consultation as soon as possible."

China is a permanent member of the Security Council.

NATO condemned the test as an "irresponsible act" that posed a grave threat to world peace.

South Korea still technically at war with the North after the 1950-53 civil war ended in a mere truce, said the size of the seismic activity indicated a nuclear explosion slightly larger than the North's two previous tests at 6-7 kilotons. The Hiroshima bomb was around 20 kilotons.

The magnitude was roughly twice as large as that of 2009, Lassina Zerbo, director of the international data centre division of the Vienna-based Comprehensive Nuclear-Test Ban Treaty Organization, said.

The US Geological Survey said that a seismic event measuring 5.1 magnitude had occurred, with North Korea later confirming the nuclear test.

"It was confirmed that the nuclear test that was carried out at a high level in a safe and perfect manner using a miniaturized and lighter nuclear device with greater explosive force than previously did not pose any negative impact on the surrounding ecological environment," KCNA said.

Despite China's tame response, it likely to be a major embarrassment for Beijing, the North's sole major economic and diplomatic ally.

"The test is hugely insulting to China, which now can be expected to follow through with threats to impose sanctions," said Mark Fitzpatrick of the International Institute for Strategic Studies think tank.

North Korea trumpeted the announcement on its state television channel to patriotic music against the backdrop of an image of its national flag.

It linked the test to its technical prowess in launching a long-range rocket in December, a move that triggered the U.N. sanctions, backed by China, that Pyongyang said prompted it to take today's action.

The North's ultimate aim, Washington believes, is to design an intercontinental ballistic missile capable of carrying a nuclear warhead that could hit the United States. North Korea says the program is aimed merely at putting satellites in space.

North Korea used plutonium in previous nuclear tests and prior to Tuesday there had been speculation it would use highly enriched uranium so as to conserve its plutonium stocks as testing eats into its limited supply of the material that could be used to construct a nuclear bomb.