An earthquake with a preliminary magnitude of 6.3 hit eastern Japan today, but no tsunami warning was issued, public broadcaster NHK said.

There were no immediate reports of damages or injuries but the quake shook buildings in the capital, Tokyo.

The US Geological Survey put the magnitude at 6.2 but later downgraded it to 5.9.

Japan's nuclear regulator said there were no irregularities at the Fukushima Daiichi nuclear plant where one of the world's worst nuclear accident unfolded after a March 2011 earthquake and tsunami.

The 2011 quake of magnitude 9 was the strongest quake ever recorded in Japan, and it generated a tsunami that knocked out the Fukushima plant, causing the worst nuclear crisis since Chernobyl a quarter of a century earlier.

About 25,000 people were by the tsunami along Japan's northeast coast.

NHK said the operator of the devastated plant, Tokyo Electric Power Co, was checking on for any impact of the quake on the plant but the nuclear regulator reported no irregularities.