Military fire trucks began dousing the fuel storage pool at reactor 3 of Fukushima I nuclear power plant, which was damaged by the earthquake and tsunami that devastated the region a week ago, public broadcaster NHK said.

Japanese military fire trucks started to cool an overheating nuclear reactor in north-eastern Japan on Friday afternoon to prevent a potential disaster, a news report said.

Military fire trucks began dousing the fuel storage pool at reactor 3 of Fukushima I nuclear power plant, which was damaged by the earthquake and tsunami that devastated the region a week ago, public broadcaster NHK said.

Steam was seen rising from the damaged building.

Seven fire trucks were to spray a total of 50 tons of water on the reactor, NHK said, citing the Defence Ministry.

Cooling systems at the reactors failed after the electricity system was knocked out in the disaster.

Five military fire-fighting vehicles late Thursday sprayed water onto the fuel storage pool inside reactor 3 after police water cannons could not get close enough earlier in the day.

U.S. military prepares team for nuclear emergency in Japan

The U.S. military has ordered 450 experts trained to handle a nuclear emergency to prepare to deploy to Japan if they are needed, a top U.S. naval official told reporters late on Thursday.

Navy Admiral Robert Willard said he would be in charge of the U.S. military assistance to Japan, which is facing a triple disaster of earthquake, tsunami and nuclear plant crisis.

An advance military team has been sent ahead to decide whether the radiological and consequence experts, as they are called, need to be called forward, Willard said.

Communication and coordination in the current crisis had been seamless, he said, thanks to close cooperation between the U.S. military and its Japanese counterpart for decades on mutual defence issues.

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