SEOUL, South Korea (AP) -- The European Mediterranean Seismological Center has detected unusual seismic activity in North Korea. It isn't immediately clear whether North Korea has conducted its fourth nuclear test.

The agency measured the magnitude of the seismic activity at 5.1 on its website on Wednesday.

South Korean government officials didn't immediately respond to calls looking to determine whether a nuclear blast or natural earthquake had taken place in the North.

North Korea conducted its third test in February 2013.

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