PYONGYANG, North Korea -- Defying U.N. warnings, North Korea on Tuesday conducted its third nuclear test in the remote, snowy northeast, taking a crucial step toward its goal of building a bomb small enough to be fitted on a missile capable of striking the United States.
North Korea said the atomic test was merely its "first response" to what it called U.S. threats, and said it will continue with unspecified "second and third measures of greater intensity" if Washington maintains its hostility.
The underground test, which set off powerful seismic waves, drew immediate condemnation from Washington, the U.N. and others. Even its only major ally, China, summoned the North's ambassador for a dressing-down.
President Barack Obama, who was scheduled to give a State of the Union address later Tuesday, said nuclear tests "do not make North Korea more secure." Instead, North Korea has "increasingly isolated and impoverished its people through its ill-advised pursuit of weapons of mass destruction," he said in a statement.
North Korea claimed the device was smaller than in previous tests; Seoul said it likely produced a bigger explosion.
The test was a defiant response to U.N. orders to shut down atomic activity or face more sanctions and international isolation. It will likely draw more sanctions from the United States and other countries at a time when North Korea is trying to rebuild its moribund economy and expand its engagement with the outside world.
Several U.N. resolutions bar North Korea from conducting nuclear or missile tests because the U.N. Security Council considers Pyongyang a would-be proliferator of weapons of mass destruction and its nuclear testing a threat to international peace and stability. North Korea dismisses that as a double standard, and claims the right to build nuclear weapons as a defense against the United States, which has been seen as enemy No. 1 since the 1950-53 Korean War. The U.S. stations more than 28,000 troops in South Korea to protect its ally.
Tuesday's test is North Korea's first since young leader Kim Jong Un took power of a country long estranged from the West. The test will likely be portrayed in North Korea as a strong move to defend the nation against foreign aggression, particularly from the U.S.
"The test was conducted in a safe and perfect way on a high level, with the use of a smaller and light A-bomb, unlike the previous ones, yet with great explosive power," North Korea's official Korean Central News Agency said, confirming speculation that seismic activity near Kilju around midday was a nuclear test.
North Korea was punished by more U.N. sanctions after a December launch of a rocket that the U.N. and Washington called a cover for a banned missile test. Pyongyang said it was a peaceful, and successful, bid to send a satellite into space.
The timing of the test is significant. It came hours before Obama's speech and only days before the Saturday birthday of Kim Jong Un's father, late leader Kim Jong Il, whose memory North Korean propaganda has repeatedly linked to the country's nuclear ambitions.
This year also marks the 60th anniversary of the signing of the armistice that ended the 1950-53 Korean War, and in late February South Korean President-elect Park Geun-hye will be inaugurated.
In Pyongyang, where it was snowing Tuesday, North Koreans gathered around televisions to watch a 3 p.m. TV broadcast announcing the nuclear test.
The test shows the world that North Korea is a "nuclear weapons state that no one can irritate," Kim Mun Chol, a 42-year-old Pyongyang citizen, told The Associated Press in the North Korean capital. "Now we have nothing to be afraid of in the world."