North Korea admitted conducting a "nuclear warhead explosion" test, hours after seismic monitors detected an artificial earthquake.

It was the fifth nuclear test the secretive state has carried out.

North Korea's state TV said the country was now capable of mounting nuclear warheads on ballistic rockets.

It said no leakage of nuclear material had occurred in the test and thatthere was no environmental impact.

The 5.3-magnitude explosion happened at 9.30am local time on Friday and had a zero depth, with an epicentre 11 miles from Sungjibaegam in the northeastern part of the county.

South Korea's president was holding a National Security Council meeting as the country's military convened a crisis management team.

Japan's Prime Minister Shinzo Abe said that a nuclear test by North Korea could not be tolerated and that Japan would protest strongly to Pyongyang.