TOKYO (AFP) - A strong 6.5 magnitude earthquake hit eastern Japan on Wednesday, according to the US Geological Survey (USGS), but local authorities said there was no risk of a tsunami.

The quake struck at 0018 GMT (8.18am on Wednesday, Singapore time) at a depth of 404 kilometres, the USGS said.

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

Fukushima operator Tokyo Electric Power (Tepco) reported that 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 Tepco 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 site 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 that it was stepping in with 47 billion yen (S$605 million) of public money to build a wall of ice underneath the plant to prevent polluted water seeping out into the sea.