"The seismic activity detected today was of magnitude 5.3, while that of the previous nuclear test was magnitude 4.9," said Yasuo Sekita, an official of the Japanese agency which monitors both weather and seismic activity.

"The impact of a tremor doubles for every 0.2 in magnitude," he said, referring to the moment magnitude scale. "This means the energy level of today's nuclear test was four times bigger than the last one."

North Korea said it staged a successful and more powerful underground nuclear test Monday than its previous test in 2006, followed later Monday by the apparent launches of three short-range missiles.

Pyongyang's moved outraged Japan, the world's only victim of atomic bomb attacks, which vowed to take "stern action" against the communist state.

The Japan Meteorological Agency said it first detected seismic activity for about 40 seconds at 9:54 am (0054 GMT) in northeastern North Korea.

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