SEOUL, Jan 6 - North Korea is "likely" to have conducted a nuclear test that caused an earthquake near a known testing site in the country, South Korea's meteorological agency said on Wednesday.

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.

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 plans a major announcement later on Wednesday, likely 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 last North Korean nuclear test, in 2013, registered at 5.1 on the U.S.G.S. scale.

North Korea is known to have conducted three nuclear tests and is under U.N. sanctions for its nuclear and missile programmes.

South Korean shares and the won currency extended declines on Wednesday.