Government authorities say an earthquake has struck southern Japan, bringing down several houses.

No tsunami warning was issued after the quake, measured at a preliminary magnitude of 6.4.

The Japan Meteorological Agency says the earthquake struck east of Kumamoto city, on the island of Kyushu.

The Sendai and Genkai nuclear plants on Kyushu are reported to be operating as normal.

No casualties have yet been confirmed though police in Kumamoto prefecture say people may be trapped in collapsed houses.

The quake struck at a depth of 10 kilometres and was followed 40 minutes later by an aftershock measuring 5.7