Radiation levels from a nuclear power plant damaged by last week's earthquake and resulting tsunami in Japan climbed to 400 millisieverts an hour briefly early Tuesday before falling to safer levels, according to reports.

The most recently observed level was 0.6 mSv an hour, equivalent to about six chest X-rays, after falling from 11.9 mSv six hours earlier, according to the International Atomic Energy Agency, which is monitoring the situation.

The IAEA said the 400 mSv radiation release could have been caused by a fire at a spent fuel storage pond near reactor number four at the the Fukushima Daiichi Nuclear Power Station. The fire has since been extinguished, Japanese authorities said.

"This is a high dose-level value, but it is a local value at a single location and at a certain point in time," the group wrote on its website. "The IAEA continues to confirm the evolution and value of this dose rate. It should be noted that because of this detected value, non-indispensable staff was evacuated from the plant, in line with the Emergency Response Plan, and that the population around the plant is already evacuated."

Workers are furiously trying to pump seawater into the affected reactors at the 40-year-old plant, which lost power after being struck by the tsunami Friday, to cool them and prevent further damage. At least three explosions have rocked the plants, due to the accumulation of hydrogen gas, the IAEA said.

The four units of the Fukushima Daini nuclear plants, which were also affected, were automatically shut down Friday. Units 1, 2 and 3 are in a safe, cold shutdown; workers are trying to restore heat-removal systems for unit 4.

“Radiation dose rate measurements observed at four locations around the plant´s perimeter over a 16-hour period on 13 March were all normal,” the IAEA said.

On Tuesday, the Japanese government warned the nearly 100,000 residents believed to live within a 20-mile radius of the Daiichi plant to stay indoors, keep the windows shut and turn off air conditioning. Already, approximately 185,000 people have been evacuated from towns near affected power plants, according to Japan's Nuclear and Industrial Safety Agency.

Around 150 people near the Daiichi site have been monitored for radiation exposure. Measures to decontaminate 23 of them have already been taken, the IAEA reported.

The crisis at the Fukushima Daiichi Nuclear Power Station, located about 155 miles northeast of Tokyo, has been called the worst reactor disaster since the Chernobyl reactor explosion almost 25 years ago. However, many experts think the risks to the public remain low.

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