An earthquake measuring 6.3 on the Richter scale was detected in North Korea after the country conducted its sixth nuclear test, a move the US and its allies in the region are likely to view as a major provocation, the media reported.
Seismological data from the US Geological Survey (USGS) showed that an explosion caused the tremor in the country's northeast, not far from the Punggye-ri nuclear test site, reports CNN.
The Korea Meteorological Administration initially put the scale of the earthquake at magnitude 5.7. and said that the seismic wave was recorded at 12.36 p.m.
If the initial data holds, it would make it the most powerful weapon that North Korea has ever tested, according to US officials.
"After analysing data provided by the Japan Meteorological Agency, the Japanese government concluded that North Korea has conducted a nuclear test," Japanese Foreign Minister Taro Kono said in a live television broadcast.
North Korean state media said it will make a major announcement at 3 p.m. (local time).
South Korea and Japan are gathering and analysing data to confirm details of the test, which Japanese Prime Minister Shinzo Abe said could not be tolerated.
"If North Korea did indeed conduct a nuclear test, we absolutely cannot tolerate and must protest firmly. We will convene a National Security council meeting to gather and analyse the information," CNN quoted Abe as saying.
South Korea's Joint Chief of Staff said all South Korean troops have been put on high alert, with the North scheduled to commemorate its founding anniversary later this week, reports Yonhap News Agency.
"(We) are closely monitoring the North Korean military's move under South Korea-U.S. coordination."
South Korea's presidential office immediately convened a National Security Council meeting on the issue.
The test was conducted after a report by North Korean state media on Sunday said that the nation's leader Kim Jong-un inspected a hydrogen bomb being loaded onto a new intercontinental ballistic missile (ICBM) after it claimed to have made a "more developed nuke".
Since 2006, North Korea has carried out five nuclear tests, including two last year.