SEOUL - 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.