North Korea nuclear test 'extremely likely' as huge tremors detected

Seismic activity detected near the country's testing centre after satellite imagery suggested a fifth nuclear trial was being prepared

A North Korean vehicle carrying a missile passes by during a mass military parade in Pyongyang's Kim Il Sung Square to celebrate the centenary of the birth of the late North Korean founder Kim Il Sung (AP)

A 5.0 magnitude quake has been detected near North Korea's north-eastern nuclear test site, raising immediate suspicions the secretive regime has tested another device.

The US Geological Survey reported an “explosion” in the area, but there was no immediate confirmation of a nuclear test, which similar seismic activity has previously indicated.

South Korea's Defence Ministry said it could not confirm the cause, and the country's weather agency said it was analysing what might have happened.

North Korea conducted a fourth nuclear test earlier this year, part of its push for a nuclear-armed missile that could one day reach the US mainland. Recent satellite imagery syggested increased activity at the Punggye-ri site, where North Korea carried out its fourth nuclear test in January.

A second test inside a year would represent a defiant response to increasing Western calls for a tightening of sanctions.

And analysts said the shockwaves made it highly likely a nuclear bomb had been tested.

The size of the seismic signal represented a device with a 20- to 30-kilotonne yield, according to one analyst, making it the largest estimated yield so far of a North Korean nuclear device.

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“That's the largest DPRK test to date, 20-30kt, at least. Not a happy day,” Jeffrey Lewis of the California-based Middlebury Institute of International Studies told Reuters, using the North's official title of the Democratic People's Republic of Korea.

“Yield estimates are always kind of approximate. The point is that it is the biggest one to date unless they revise the yield downward,” he said.

Friday morning's seismic activity tremor put regional governments on immediate alert.

A Japanese government spokesman said there was a high possibility Pyongyang had conducted a nuclear test.

Prime Minister Shinzo Abe has instructed the relevant ministries to collect and analyse information, said Chief Cabinet Secretary Yoshihide Suga at a news conference.

South Korea's presidential Blue House was also expected to hold a National Security Council meeting.