Military helicopters dropped water Thursday onto the damaged nuclear reactor in north-eastern Japan and water cannons and firefighting trucks were also dispatched to douse the site.

The aircraft dumped water four times into the overheated fuel-storage pool of reactor 3 at the power plant in Fukushima, where the cooling systems for its six reactors failed in the wake of Friday's earthquake and tsunami.

Elevated radiation levels on Wednesday forced authorities to postpone their plans to deploy the helicopters, an operation requested by the plant's operator, the Tokyo Electric Power Co (TEPCO).

Japanese Defense Minister Toshimi Kitazawa said radiation levels had dropped to 4.13 millisievert per hour at an altitude of 300 meters and 87.7 millisievert at 90 metres.

On Tuesday night, alarming levels of radiation were taken at the plant, with measurements reaching 1,000. According to the government, this is a level that can cause radiation poisoning.

Eleven military firefighting vehicles would start to douse the reactor in the afternoon and more vehicles were also on the way, Kitazawa said as TEPCO and authorities tried to lower the temperatures and pressure building in the reactors.

Police were planning to start using a water-cannon truck in the afternoon to help contain the disaster.

TEPCO said it would install a new power line to restore electricity in the complex. The new line will hopefully enable pumps to send water to the reactors and pools to keep them cool, a TEPCO official said.

About 50 nuclear plant workers have stayed behind to try to cool down the plant's six reactors.

U.S. President Barack Obama talked to Prime Minister Naoto Kan Thursday about the ongoing crisis, promising to dispatch more experts to Japan, said Yukio Edano, chief cabinet secretary.

It is still unclear whether the inner containment shell of reactor number 3 has been damaged. Edano said Wednesday that there was no definitive information about the
containment shell but conceded that radioactive steam may have leaked.

The shell is one of three containment structures in the reactor designed to prevent the leakage of radioactive materials.

Meanwhile, the US embassy in Tokyo advised American citizens to move at least 80 kilometers from the plant in Fukushima, 250 kilometers north of Tokyo.

Japanese authorities have called for the evacuation of residents within 20 kilometers of the damaged nuclear plant, and have told residents within 30 kilometers to stay indoors.