A strong quake was felt this afternoon in Tokyo, shaking buildings in Japan's capital four days after a massive tremor sparked a devastating tsunami that ravaged the country's northeast coast.

The Japan Meteorological Agency put the magnitude of the quake at 6.0.

The epicentre was located in Shizuoka prefecture, about 120 kilometres (75 miles) southwest of the capital, and near Mount Fuji, which is prone to earthquakes.

The quake occured at a relatively shallow depth of 10 kilometres.

Meanwhile, the levels of radiation detected in Tokyo and neighbouring Chiba fell back after rising above normal levels possibly due to radiation emitted from a quake-hit nuclear power plant, officials said.

Tokyo officials said they detected 0.809 of a micro-sievert between 10:00 and 11:00 am in the capital, some 250 kilometres (155 miles) southwest of the troubled Fukushima plant on the Pacific coast.

The normal level is about 0.035-0.036 micro-sieverts. A chest X-ray typically involves a dose of 20 micro-sieverts.

The level fell to an average 0.075 micro-sieverts four hours later.

"It is returning closer to the normal level. But we need to keep a close watch on it," city health and welfare official Keiichi Nakaya said.

City government official Sairi Koga said even the highest reading is not considered to be "at a level where the human body is affected".

The central government warned Tuesday that radiation levels near the quake-stricken nuclear plant were now harmful to human health, following two explosions and a fire earlier in the day.

In Chiba, southwest of Tokyo, the level of radiation surged to 0.313 micro-sieverts at 5:00 pm, the highest reading since the quake struck on Friday, the city government said.

But it fell back to 0.039 four hours later, compared with the range of 0.022-0.044 recorded in 2009, it said.

"The figures, including the maximum, do not represent a level which affects human health," it said in a press release.