U.S. President Barack Obama said North Korea's third nuclear test, in defiance of U.N. resolutions, was a threat and a provocation and that the United States would lead the world in responding. North Korea has said Tuesday's test was an act of self-defense against "U.S. hostility" and threatened stronger steps if necessary.
"Provocations of the sort we saw last night will only isolate them further, as we stand by our allies, strengthen our own missile defense, and lead the world in taking firm action in response to these threats," Obama said in his State of the Union address, delivered 24 hours after the North's test.
North Korea tested nuclear devices in 2006 and 2009. But despite the three tests and a long-range rocket launch, it is not believed to be close to manufacturing a nuclear missile capable of targeting the United States.
But Washington believes the isolated state's ultimate aim is to design an intercontinental ballistic missile capable of carrying a nuclear warhead that could reach the continental United States. North Korea says its rocket program is aimed at putting satellites in space.
The latest test has drawn condemnation from around the world, including from China which for years has been the North's only major ally.
The U.N. Security Council held an emergency meeting at which its members, including China, "strongly condemned" the test and vowed to start work on appropriate measures in response, the president of the council said.
North Korean leader Kim Jong-un, the third member of his family to rule, has presided over two long-range rocket launches and a nuclear test during his first year in power, pursuing policies that have propelled his impoverished and malnourished country closer to becoming a nuclear weapons power.
North Korea said the test had "greater explosive force" than those in 2006 and 2009. Its KCNA news agency said it had used a "miniaturized" and lighter nuclear device, indicating it had again used plutonium, which is suitable for use as a missile warhead.
China, which has shown signs of increasing exasperation with the recent bellicose tone of its reclusive neighbor, summoned the North Korean ambassador in Beijing and protested sternly, the Chinese 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".
Analysts said the test was a major embarrassment to China, which is a permanent member of the Security Council and North Korea's sole major economic and diplomatic ally.
Obama said the test would be a setback for North Korea's economic development. "The regime in North Korea must know that they will only achieve security and prosperity by meeting their international obligations," he said.
U.S. ambassador to the United Nations Susan Rice said Washington and its allies intended to "augment the sanctions regime" already in place due to Pyongyang's previous atomic tests. 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.
Russian Foreign Minister Sergei Lavrov urged North Korea to abandon its nuclear arms program and return to talks. NATO condemned the test as an "irresponsible act."
South Korea, still technically at war with North Korea after a 1950-53 civil war ended in a truce, also denounced the test. Obama spoke to South Korean President Lee Myung-bak on Tuesday and told him the United States "remains steadfast in its defense commitments" to South Korea, the White House said.