A strong but deep 6.6-magnitude earthquake struck off the coast of southwest Japan early Sunday, the US Geological Survey said, but no tsunami warning was issued.

There were no immediate reports of injury or damage caused by the earthquake; its epicentre was 135 kilometres (84 miles) northwest of Naze, Japan.

The quake, which USGS originally recorded as a 6.7-magnitude temblor, was relatively deep at 160 kilometres. 

Japan's Kyodo News agency reported that the earthquake was felt in Okinawa as well as the island of Kyushu.

It struck at 12:51 am local time (1551 GMT Saturday), according to USGS and was followed by a shallower 4.4-magnitude quake two hours later off Japan's southeast coast.

Another shallow quake, registering 5.3 in magnitude, occurred at 2018 GMT Saturday in the waters between southwest Japan and Taiwan. No tsunami warnings were issued.

Japan sits on the Pacific "Ring of Fire", an arc of intense seismic activity that stretches through Southeast Asia and across the Pacific basin.

In 2011, a devastating 9.0-magnitude earthquake struck roughly 130 kilometres east of Miyagi prefecture, unleashing an enormous tsunami, triggering the Fukushima nuclear reactor meltdown and killing nearly 16,000 people.