North Korea conducted its fifth and largest nuclear test on Friday, South Korean officials and analysts said after meteorological agencies reported a surface-level seismic tremor near the North's known test site.
US, European and Chinese agencies reported that the tremor was recorded at 0030 GMT, measuring the tremor at a magnitude of 5.3.
South Korea called an emergency meeting of its National Security Council on suspcion that the North had conducted a nuclear test, and China's environment ministry began emergency radiation monitoring along its borders with North Korea in northeast China.
Japanese Prime Minister Shinzo Abe said such a nuclear test could not be tolerated. Japan's foreign minister lodged a protest and Tokyo also sent two military jets to begin measuring for radiation.