TOKYO (REUTERS) - Japan has concluded that the tremors detected in North Korea on Sunday (Sept 3) were a nuclear explosion, marking the sixth atomic test by Pyongyang since 2006.

"After examining the data we concluded that it was a nuclear test," Foreign Minister Taro Kono said at a briefing broadcast by public broadcaster NHK following a meeting of Japan's National Security Council.

Japan's Ministry of Defence said it had dispatched at least three military jets from bases in Japan to test for radiation.

Japanese Prime Minister Shinzo Abe said a sixth nuclear test by Pyongyang would be "absolutely unacceptable," after a 6.3 magnitude explosion in the North indicated a new detonation.

"If it forcibly conducted a nuclear test, it's absolutely unacceptable. We have to strongly protest it," Abe said.

"There is a possibility that this is not a natural quake and that North Korea conducted a nuclear test," he said, adding that the Japanese weather agency detected a seismic wave.