An earthquake with a magnitude of 6.6 jolted southwestern Japan early on Saturday morning, injuring 13 people, the authorities and local media said.

No tsunami warning was issued after the quake struck with an epicentre 45 kilometres deep at 1:08am off the coast of Kyushu, the southernmost of Japan's four main islands, the Japan Meteorological Agency (JMA) said.

The quake caused shaking in Oita and Miyazaki prefectures that measured five+ on Japan's seismic intensity scale, which has a maximum of seven, the agency said.

Thirteen people were injured in nearby regions, including two people in their 80s who were seriously hurt, the Yomiuri newspaper reported, citing local authorities.

Multiple reports of damage to buildings, water pipes and roads were confirmed, said public broadcaster NHK.

No abnormalities were reported at the Ikata nuclear power plant, operated by Shikoku Electric Power, or the Sendai plant operated by Kyushu Electric Power in southern Japan, the Nuclear Regulation Authority said.

"In the past, 10 per cent to 20pc of strong earthquakes were followed by a quake of the same level, so be aware of another quake of up to five+ intensity scale in regions that experienced large jolts, for around a week," the JMA said in a statement.