Tokyo Electric executives said Thursday that they believed the rods in that pool were covered with water, but an official with Japan's nuclear safety agency later expressed skepticism about that and moved closer to the U.S. position.

"Considering the amount of radiation released in the area, the fuel rods are more likely to be exposed than to be covered," Yuichi Sato said.

Emergency workers were forced to temporarily retreat from the plant Wednesday 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.

U.S. officials were taking no chances, and Prime Minister Naoto Kan and U.S. President Barack Obama spoke about the crisis early Thursday.

In a statement, U.S. Ambassador John V. Roos made his evacuation recommendation "in response to the deteriorating situation" at the Fukushima complex. In Washington, the State Department warned U.S. citizens to consider leaving the country, and offered voluntary evacuation to family members and dependents of U.S. personnel in the cities of Tokyo, Yokohama and Nagoya.

This screengrab taken off Japanese national broadcaster NHK shows a Japanese military twin-rotor cargo helicopter dumping water onto reactor no. 3 (C) at the stricken Fukushima nuclear power plant on March 17, 2011. [AFP]

Chartered planes also would be brought in to help private American citizens who wished to leave, the State Department said.

While American officials have been careful not to criticize Japan's response, they have made clear it's difficult to ascertain what is going on.

"It's a very fluid and indeed it's a very confused situation," U.S. Deputy Energy Secretary Daniel Poneman told reporters Wednesday.

Japanese officials raised hopes of easing the crisis early Thursday, saying they may be close to bringing power back to the plant. The new power line would revive electric-powered pumps, making it easier for workers to control the high temperatures.

Tokyo Electric officials said they hoped to have the new 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.

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

Along the tsunami-savaged coast, people must stand in line for food, gasoline and kerosene to heat their homes. In the town of Kesennuma, they lined up to get into a supermarket after a delivery of key supplies, such as instant rice packets and diapers.

Each person was only allowed to buy 10 items, NHK television reported.

With diapers hard to find in many areas, an NHK program broadcast a how-to session on fashioning a diaper from a plastic shopping bag and a towel.

More than 5,300 people are officially listed as dead, but officials believe the toll will climb to well over 10,000.

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 northeastern coast.