A strong 6.0-magnitude earthquake has struck off Japan's southern island of Okinawa, the US Geological Survey says.

The quake struck at 4:19am (local time), 165 kilometres north-west of Okinawa's capital Naha at a depth of 138 kilometres.

There were no immediate reports of casualties or damage.

About 20 per cent of the world's most powerful earthquakes strike Japan.