Third test in seven years || North Korea conducted an underground nuclear test; South Korea on high alert
The test is another big step toward North Korea's goal of building a warhead that can be mounted on a missile.
North Korea conducted its third-ever nuclear test on Tuesday, a move likely to anger its main ally China and increase international action against Pyongyang and its new young leader, Kim Jong-un.
U.S. President Barack Obama said North Korea's latest nuclear test was a "highly provocative act" that hurt stability in the region and called its nuclear program a threat to U.S. and international security.
"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.
"We will strengthen close coordination with allies and partners and work with our Six-Party partners, the United Nations Security Council, and other UN member states to pursue firm action," he said.
UN Secretary-General Ban Ki-moon condemned North Korea's test, saying it was a "clear and grave violation" of UN Security Council resolutions.
South Korea said the size of the seismic activity indicated a nuclear explosion slightly larger than the North's two previous tests at 6-7 kilotons, although that is still relatively small. The Hiroshima bomb was around 20 kilotons.
The U.S. Geological Survey said that a seismic event measuring 5.1 magnitude had occurred on Tuesday, 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.
The test prompted the UN Security Council to call for an emergency meeting later on Tuesday and came as China celebrated the lunar new year, potentially increasing embarrassment for Beijing, the North's sole major economic and diplomatic ally.
"I think it will be proven to be a self-defeating and self-suffocating blunder on the part of the DPRK (Democratic People's Republic of Korea)," an Asian diplomat to the United Nations told Reuters in New York.
"They have chosen the worst timing to conduct this testing ... This will also be an open invitation to the international community to up the ante to corner the DPRK."
It may take days to ascertain whether the North used highly enriched uranium for the first time in the nuclear test, a move that would give it a second path to a nuclear weapon.
North Korea has used plutonium in previous tests and needs to conserve its stocks as testing eats into its limited supply of the material that could be used to construct a nuclear bomb.
The Vienna-based Comprehensive Nuclear-Test Ban Treaty Organization, the international atomic test monitor, said the event had hallmarks similar to the North's previous nuclear tests in 2006 and 2009.
"This act would constitute a clear threat to international peace and security, and challenges efforts made to strengthen global nuclear disarmament and non-proliferation," it said.
Japan immediately called for sanctions against North Korea whose December long-range rocket launch prompted new UN sanctions that Pyongyang said earlier would push it to undertake a third nuclear test.
South Korea's defense ministry said additional nuclear tests and rocket launches by the North should not be ruled out.
South Korea's Yonhap news agency said Pyongyang had informed China and the United States of its plans to test on Monday.
When new leader Kim Jong-un took office after his father's death in December 2011, there were hopes the youthful leader would bring economic reforms and end his father Kim Jong-il's "military first" policies that have seen the North declare itself a "nuclear weapons state".
Since taking office however, he has purged the military, pushed ahead with two long-range rocket launches, which critics say breach UN sanctions.
Tuesday's action appeared to have been timed for the run-up to Feb. 16 anniversary celebrations of Kim Jong-il's birthday, as well as to achieve maximum international attention.
But options for the international community appear to be in short supply, as North Korea is already one of the most heavily sanctioned states on earth.
Significantly, the test comes at a time of political transition in China, Japan and South Korea, and as U.S. President Barack Obama begins his second term.
Japanese Prime Minister Shinzo Abe is bedding down a new government and South Korea's new president, Park Geun-hye, prepares to take office on February 25.
China too is in the midst of a once in a decade leadership transition to Xi Jinping, who takes office in March.
But the longer term game plan from Pyongyang may be to restart talks aimed at winning aide for its impoverished and stricken economy that is smaller than it was 20 years ago.
Its puny economy and small diplomatic reach means the North struggles to win attention on the global stage - other than through nuclear tests and attacks on South Korea, last made in 2010.
"Now the next step for North Korea will be to offer talks. They will either offer to restart six-party talks or military talks - any form to start up discussion again to bring things to their advantage," said Jeung Young-tae, senior research fellow at the Korea Institute for National Unification in Seoul.