SEOUL, South Korea – North Korea successfully detonated a miniaturized nuclear device at a northeastern test site Tuesday, state media said, defying U.N. Security Council orders to shut down atomic activity or face more sanctions and international isolation.

The underground explosion could take North Korea a big step closer to its goal of building a nuclear warhead small enough to be mounted on a long-range missile that could threaten the United States.

Official state media said the test was conducted in a safe manner and is aimed at coping with "outrageous" U.S. hostility that "violently" undermines the North's peaceful, sovereign rights to launch satellites. North Korea faced sanctions after a December launch of a rocket the U.N. and Washington called a cover for a banned missile test.