North Korea is "likely" to have conducted a nuclear test on Wednesday that caused an earthquake near a known testing site in the isolated country, the South Korean and Japanese governments said.

The United States Geological Survey reported a 5.1 magnitude quake that South Korea said was 49 km (30 miles) from the Punggye-ri site where the North has conducted nuclear tests in the past.

"We suspect a man-made earthquake and are analysing the scale and epicentre of the quake," a Korea Meteorological Administration official told Reuters by phone.

North Korea plans a major announcement at 0330 GMT, South Korean media said.

South Korea's presidential office convened an emergency security meeting while Japan's chief government spokesman said the earthquake was likely caused by a nuclear test.

The U.S. Defense Department is "looking into reports of a possible seismic event near North Korea's nuclear facilities," a U.S. official said.

While the USGS put the depth of the earthquake at 10 km, the South Korean agency said it was near the surface. The earthquake was detected just after 10 a.m. Seoul time (0100 GMT).

North Korea, under U.N. sanctions for its nuclear and missile programmes, has so far conducted three nuclear tests - in 2006, 2009 and 2013 - all at Punggye-ri, near where the earthquake took place.

The 2013 test registered at 5.1 on the USGS scale.

It is not yet known if North Korea has successfully miniaturised a nuclear device small enough to be used as a warhead on a ballistic missile, but the likelihood of the isolated country successfully miniaturising a device increases with each test.

South Korean stocks and the won currency fell slightly after reports of the likely test on Wednesday, and foreign exchange authorities were suspected by dealers to have intervened.