North Korean state television said the test elevated the country’s nuclear arsenal and was part of its response to international sanctions following its earlier nuclear test and long-range rocket launch in January and February.
Experts believe is the fifth and most powerful test Pyongyang has conducted so far.
South Korea and Japan say meteorological agencies reported a surface-level seismic tremor near the North’s known test site.
Indications are this is the most powerful yet.
How strong was the tremor?
The US Geological Survey and European agencies measured a magnitude of 5.3 at 0030 GMT.
Agencies in China, Pyongyang’s main diplomatic ally, also recorded the tremor.
The biggest test to date
Tests of seismic magnitude suggest this is the most powerful nuclear test conducted by North Korea to date.
Jeffrey Lewis of the California-based Middlebury Institute said surface level recordings indicate a blast with a 20 to 30 kilotonne yield.
This would make Friday’s reported test larger than the nuclear bomb dropped by the US on the Japanese city of Hiroshima in the Second World War.
It would also be potentially bigger than the one dropped on Nagasaki soon afterwards.
South Korean President Park Guen-Hye, who is in Laos for a summit of Asian leaders, said it was a nuclear test.
Park said North Korean leader Kim Jong-Un was showing “maniacal recklessness” in completely ignoring the world’s call to abandon his pursuit of nuclear weapons.
Seoul’s presidential office said Barack Obama spoke with Park by telephone as he flew back from Laos.
Obama held unscheduled meetings with Park and Japanese Prime Minister Shinzo Abe after the missile tests.
He said Washington needed to maintain a sense of urgency within the international community on sanctions against Pyongyang.
Japan also described it as a nuclear test.
Prime Minister Abe said a nuclear test could not be tolerated. Japan’s foreign minister also lodged a protest while two military jets have been sent to begin monitoring radiation.
China‘s environment ministry has begun emergency radiation monitoring along its border with North Korea, according to state television reports. (Euronews)