Tokyo: Six people were injured on Monday after an explosion at a nuclear reactor in northeast Japan following an earthquake, media reports said.

Fresh white smoke rose from Japan's Fukushima Daiichi building after an explosion shook the quake-damaged Japanese nuclear power plant.

Japan's nuclear safety agency said the blast, at the number 3 reactor at the Fukushima No 1 plant, was believed to be caused by hydrogen.

"We believe it was a hydrogen explosion. It is not immediately known if it affected the reactor," said spokesman Ryo Miyake.

A wall of the building collapsed due to the blast, according to Japanese public broadcaster which showed plumes of smoke above the plant..

The city is home to 10 reactors at two power plants, and explosions have occurred at two of the reactors since Friday's magnitude-9 earthquake and tsunami. One happened Saturday and the second Monday after a magnitude-6.2 aftershock.

A TEPCO spokesman said radiation was measured at the plant at 20 microsievert per hour 25 minutes after Monday's blast. Japan allows an hourly exposure of 500 microsievert, which is a measure of the biological effects of radiation.

5.8-magnitude quake hits again

An offshore quake struck 140 kilometres northeast of Tokyo on Monday morning, shaking tall buildings in Japan's capital, but authorities did not issue a tsunami alert.

There were no immediate reports of damage or casualties.

The quake off coastal Ibaraki prefecture -- one of many aftershocks since Friday's massive 8.9 quake -- had a 5.8-magnitude, said the US Geological Survey, which said the quake struck at a depth of 18 kilometres.