Japanese top government spokesman, Yoshihide Suga, confirmed on Friday (Sep 9) North Korea carried out nuclear tests, causing 5.3 magnitude tremors to occur near a known test site.
"Taking into consideration the irregularity of the tremors, and the information we have gathered, we have come to the conclusion that North Korea did indeed carry out a nuclear test," said Suga. "This is of course something we cannot tolerate as a country, and we immediately expressed our strongest denunciation and protest through embassy channels in Beijing."
This would be the North's fifth and largest nuclear test, South Korean officials and analysts said after meteorological agencies reported a surface-level seismic tremor near the test site.
Japan's Defence Minister Tomomi Inada described the nuclear test, combined with advances made by the Pyongyang regime in mobile ballistic technology, as posing a grave threat to Japan.
"We can't deny the possibility that North Korea is miniaturising a device to build a warhead," Inada said at a press briefing in Tokyo, adding that the magnitude of the tremor suggested it was not a test of a hydrogen device.
Jeffrey Lewis at the California-based Middlebury Institute of International Studies said the highest estimates of seismic magnitude suggested this was the most powerful nuclear test conducted by the North so far. He said the seismic magnitude and surface level indicated a blast with a 20- to 30-kilotonne yield. Such a yield would make this test, if confirmed, larger than the nuclear bomb dropped on the Japanese city of Hiroshima in World War Two and potentially bigger than that dropped on Nagasaki soon after.
North Korea has been under UN Security Council sanctions since it first tested an atomic device in 2006.