The quake struck at 0018 GMT at a depth of 404 kilometres (251 miles), the USGS said.

"The epicentre is in the Pacific, hundreds of kilometres (miles) south of Tokyo. We see no risk of a tsunami," a spokesman for the Japanese weather agency said.

Fukushima operator TEPCO reported there were no new problems at the stricken nuclear plant.

The quake, measured at 6.9 by Japanese seismologists, was centred on a spot more than 600 kilometres south of Tokyo, the USGS said.

AFP journalists in the Japanese capital reported feeling a long, rumbling quake that shook buildings. They said it was the largest they had felt in the quake-prone city for some time.

A spokesman for Tokyo Electric Power said the earthquake did not cause any additional damage at Fukushima Daiichi nuclear plant, the site of the worst nuclear accident in a generation where radioactive waste water has been leaking into the Pacific Ocean.

"We have confirmed that there was no immediate abnormality," according to data collected by monitoring equipment, a TEPCO spokesman said, adding that crews will patrol the crippled plant's vast campus to survey whether any physical damage has been caused.

A series of leaks of radioactive water at the nuclear plant has left TEPCO on the back foot in recent months.

On Tuesday the Japanese government announced it was stepping in with 47 billion yen ($470 million) of public money to build a wall of ice underneath the plant to prevent polluted water seeping out into the sea.