SEOUL (Reuters) - Seismic activity has been detected in North Korea with an earthquake measuring 4.9 magnitude registered by the U.S. Geological Survey, in a move that South Korea said indicated that Pyongyang had carried out a third nuclear test.

The quake occurred at 11:57 a.m Korean time (0257 GMT) and South Korea's presidential office said that it was "likely" a nuclear test, according to the South's Yonhap news agency

North Korea is not prone to seismic activity.

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

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