A magnitude-6.8 earthquake has hit an ocean area east of Iwaki city on the Japanese island of Honshu, the US Geological Survey (USGS) has said.

It hit 131km (81 miles) off Iwaki, at a depth of 10km, the USGS said. A tsunami warning has been issued.

In 2011 the north-east of Japan was hit by one of the most powerful earthquakes since records began, triggering a massive tsunami.

Reactors at the Fukushima nuclear plant were damaged at the time.

The NHK public broadcaster has reported that staff at the Fukushima plant are checking to see if there is any damage from this latest quake. It later reported that there were no immediate abnormalities.

The quake registered a moderate four on the Japanese seismic scale of 1-7, correspondents say, meaning that major damage is unlikely.

The magnitude-9.0 quake that struck on 11 March 2011 caused a devastating tsunami and left more than 15,000 people dead, with at least 3,200 missing.

That quake triggered a meltdown of fuel rods at the Fukushima nuclear plant, causing radiation leaks and mass evacuations.