Military helicopters dumped loads of seawater onto Japan's stricken nuclear complex today, turning to combat-style tactics while trying to cool overheated uranium fuel that may be on the verge of spewing out more radiation.

Plant operators also said they were racing to finish a new power line that could restore cooling systems and ease the crisis at the Fukushima Dai-ichi plant on the country's north-east coast.

The top US nuclear regulatory official gave a far bleaker assessment of the situation than the Japanese, and the US ambassador said the situation was "deteriorating" while warning US citizens within 50 miles of the complex to leave the area or at least remain indoors.

The Japanese government said it had no plans to expand its mandatory, 12-mile exclusion zone around the plant on the north-east coast, while also urging people within 20 miles to stay inside.

The crisis at the nuclear complex was set off when last week's 9.0-magnitude earthquake and tsunami knocked out power and destroyed backup generators needed for the reactors' cooling systems, adding a major nuclear crisis for Japan as it struggled with twin natural disasters that killed more than 10,000 people and left hundreds of thousands homeless.

Two Japanese military CH-47 Chinook helicopters began dumping seawater on the complex's damaged Unit 3 at 9:48am (0048 GMT), defence ministry spokeswoman Kazumi Toyama said. The choppers dumped at least four loads on the reactor in just the first 10 minutes, though television footage showed much of it appearing to disperse in the wind.

Chopper crews were flying missions of about 40 minutes each to limit their radiation exposure, passing over the reactor about four times each with loads of about about 2,000 gallons of water.

The water drops were aimed at cooling the Unit 3 reactor, as well as replenishing water in that unit's cooling pool, where used fuel rods are stored, Toyama said. The plant's owner, Tokyo Electric Power Co., said earlier that pool was nearly empty, which would cause the rods to overheat and emit even more radiation.

Defence Minister Toshifumi Kitazawa told reporters that emergency workers had no choice but to try the water dumps before it was too late.

US officials, meanwhile, said Unit 4 also was seriously at risk.

US Nuclear Regulatory Commission chairman Gregory Jaczko said at a congressional hearing in Washington that all the water was gone from that unit's spent fuel pool. Mr Jaczko said anyone who gets close to the plant could face potentially lethal doses of radiation.

"We believe radiation levels are extremely high," he said.

Japanese officials earlier had expressed similar worries about the fuel pool, but yesterday afternoon they said the rods in the pool were believed to be covered. However, with much of the monitoring equipment in the plant inoperable it was impossible to be sure of the situation.

"We haven't been able to get any of the latest data at any spent fuel pools. We don't have latest water levels, temperatures, none of the latest information," Masahisa Otsuki, a Tokyo Electric Power official said yesterday.

Chief Cabinet Secretary Yukio Edano said that along with the helicopter water drops, special police units would use water cannons - normally used to quell rioters - to spray water onto the Unit 4 storage pool. The high-pressure water cannons will allow emergency workers to stay farther away.

Emergency workers were forced to temporarily retreat from the plant yesterday when radiation levels soared, losing precious time. While the levels later dropped, they were still too high to let workers get close.

The storage pools need a constant source of cooling water. Even when removed from reactors, uranium rods are still extremely hot and must be cooled for months, possibly longer, to prevent them from heating up again and emitting radioactivity.

A core team of 180 emergency workers has been at the forefront of the struggle at the plant, rotating in and out of the complex to try to reduce their radiation exposure.

But experts said that anyone working close to the reactors was almost certainly being exposed to radiation levels that could, at least, give them much higher cancer risks.

"I don't know any other way to say it, but this is like suicide fighters in a war," said Keiichi Nakagawa, associate professor of the Department of Radiology at University of Tokyo Hospital.

Experts note, though, that radiation levels drop quickly with distance from the complex. While elevated radiation has been detected well outside the evacuation zone, experts say those levels are not dangerous.

Japanese officials raised hopes of easing the crisis this morning, saying they may be close to bringing power back to the plant and restoring the reactors' cooling systems.

The new power line would revive electric-powered pumps, making it easier for workers to control the high temperatures that may have led to partial meltdowns in three reactors. The company is also trying to repair its existing disabled power line.

Tokyo Electric officials said they hoped to have the power line working later Thursday, and had electricians standing by to connect the power plant.

Nearly a week after the disaster, police said more than 452,000 people were staying in schools and other shelters, as supplies of fuel, medicine and other necessities ran short. Both victims and aid workers appealed for more help.

More than 4,300 people are officially listed as dead, but officials believe the toll will climb to well over 10,000.

"There is enough food, but no fuel or gasoline," said Yuko Niuma, 46, as she stood looking out over Ofunato harbour, where trawlers were flipped on their sides.

The threat of nuclear disaster only added to Japanese misery.

"The anxiety and anger being felt by people in Fukushima have reached a boiling point," the governor of Fukushima prefecture, Yuhei Sato, fumed in an interview with the Japanese television network NHK. He said evacuation preparations were inadequate, saying centres lacked enough hot meals and basic necessities.

The chief of the UN nuclear agency, Yukiya Amano, said he would go to Japan to assess what he called a "very serious" situation and urged Tokyo to provide better information to his organisation.

Other countries have complained that Japan has been too slow and vague in releasing details about its rapidly evolving crisis at the complex of six reactors along Japan's north-eastern coast.