North Korea successfully detonated a ‘miniaturised’ nuclear device at a northeastern test site on 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.

The North said it used a “lighter, miniaturised atomic bomb” that still has more explosive force than past tests.

Experts say regular tests are needed to perfect North Korea’s goal of building nuclear warheads small enough to be placed on long-range missiles. This atomic test, North Korea’s third since 2006, is expected to take Pyongyang closer to possessing nuclear-tipped missiles designed to strike the United States.

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