North Korea confirmed Tuesday it had "successfully" carried out an underground nuclear test, drawing immediate condemnation from rival South Korea.

"A third nuclear test has been successfully staged," the state-run Korean Central News agency said, as cited by international media.

"The nuclear test was conducted as part of measures to protect our national security and sovereignty against the reckless hostility of the United States that violated our republic's right for a peaceful satellite launch," KCNA said.

Earlier on Tuesday, an earthquake shook North Korea prompting reports of a possible nuclear test.

South Korea's Defense Ministry estimates the power of the presumed blast at 10 Kiloton.

Seismic activity has been detected in North Korea with a 5.0-magnitude earthquake at 11:58 a.m Korean time (02:58 GMT) on Tuesday.

The quake's epicenter was only one km deep, and was close to the North's known nuclear test site.

The Comprehensive Nuclear Test Ban Treaty Organization (CTBTO) said that the seismic event in North Korea showed "clear explosion-like charateristics."

"The event shows clear explosion-like characteristics and its location is roughly congruent with the 2006 and 2009 DPRK (North Korea) nuclear tests," CTBTO Executive Secretary Tibor Toth said in a statement.

North Korea gave the United States and China an advance notice of conducting a nuclear test the day before.

North Korea vowed on January 23 to strengthen its military capabilities and step up its controversial nuclear program. The announcement came as a response to the UN Security Council's new sanctions on the country over the December rocket launch.

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