TOKYO (AFP) - The levels of radiation detected in Tokyo and neighboring Chiba fell back Tuesday 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 kilometers (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.