North Korea successfully detonated a miniaturized nuclear device at a northeastern test site Tuesday, state media said, defying UN 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 the United States would find threatening.

Official state media said the test was conducted in a safe manner and is aimed at coping with "outrageous" US 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 UN and Washington called a cover for a banned missile test.

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

The United States Geological Survey said earlier Tuesday that it had detected a 4.9 magnitude earthquake in North Korea.

The nuclear test is North Korea's first since leader Kim Jong Un took power in December 2011 following the death of his father, Kim Jong Il, and marks a bold statement for the young leader as he unveils his domestic and foreign policy for a country long estranged from the West.

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.

"The scientific field for national defence of the DPRK succeeded in the third underground nuclear test at the site for underground nuclear test in the northern part of the DPRK on Tuesday. The test was carried out as part of practical measures of counteraction to defend the country's security and sovereignty in the face of the ferocious hostile act of the US which wantonly violated the DPRK's legitimate right to launch satellite for peaceful purposes", read the statement by North Korea's official news agency.