A seismic tremor was recorded in an area around North Korea's known nuclear site on Friday, and was suspected to be the fifth nuclear test by the isolated nation, South Korea's Yonhap news agency said.
U.S., European and Chinese seismological agencies reported that the tremor was recorded at 0030 GMT at surface level.
A South Korean government official said it was a suspected nuclear test, Yonhap reported, and Seoul called for an emergency National Security Council meeting.
The China Earthquake Networks Center said the tremor was a suspected explosion.
Japan's top government spokesman also said there was a high possibility the seismic event indicated that North Korea had conducted a nuclear test and that relevant ministries had been instructed to gather information.
Two months ago, U.S.-based 38 North, a North Korea monitoring project, said satellite images showed a high level of activity at North Korea's nuclear test site, called Punggye-ri.
Speculation has intensified that North Korea may conduct a fifth nuclear test after the United States blacklisted its leader, Kim Jong Un, on July 6 for human rights abuses.
North Korea conducted its fourth nuclear test in January, resulting in tough new U.N. sanctions. It has conducted a series of missile tests this year, including a submarine-launched missile.
The January test was recorded with a magnitude of 5.1, according to the U.S. Geological Survey.
North Korea has been under U.N. Security Council sanctions since it first tested an atomic device in 2006.