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

The quake struck at 4:19 am (1919 GMT Thursday), 165 kilometres (105 miles) northwest of Okinawa's capital Naha at a depth of 138 kilometres, according to the USGS.

There were no immediate reports of casualties or damage.

Around 20 percent of the world's most powerful earthquakes strike Japan.