Japanese military helicopters and fire trucks doused an overheating nuclear plant with water on Thursday while the United States said it was sending aircraft to help Americans worried about spreading radiation leave the country
Engineers tried to run power from the main grid to start water pumps needed to cool two reactors and spent fuel rods considered to pose the biggest risk of spewing radioactivity into the atmosphere.
U.S. officials expressed alarm about leaking radiation but took pains not to criticize Japan's government, which appears overwhelmed by the crisis. Washington's actions indicated a divide with its close ally about the preciousness of the world's worst nuclear accident since the Chernobyl disaster in 1986.
The top U.S. nuclear regulator said the cooling pool for spent fuel rods at reactor No.4 may have run dry and another was leaking.
Gregory Jaczko, head of the Nuclear Regulatory Commission, told a congressional hearing that radiation levels around the cooling pool were extremely high, posing deadly risks for workers still toiling in the wreckage of the earthquake-shattered power plant.
"It would be very difficult for emergency workers to get near the reactors. The doses they could experience would potentially be lethal doses in a very short period of time," he said in Washington.
Japan's nuclear agency said it could not confirm if water was covering the fuel rods. The plant operator said it believed the reactor spent-fuel pool still had water as of Wednesday, and made clear its priority was the spent-fuel pool at the No.3 reactor.
On Thursday morning alone, military helicopters dumped around 30 tonnes of water, all aimed at this reactor. One emergency crew temporarily put off spraying the same reactor with a water cannon due to high radiation, broadcaster NHK said, but another crew later began hosing it.
Health experts said panic over radiation leaks from the Daiichi plant, around 240 km (150 miles) north of Tokyo, was diverting attention from other life-threatening risks facing survivors of last Friday's earthquake and tsunami, such as cold, heavy snow in parts and access to fresh water.
Inside the complex, torn apart by four explosions since a 9.0-magnitude earthquake and tsunami hit last Friday, workers in protective suits and using makeshift lighting tried to monitor what was going on inside the six reactors. They have been working in short shifts to minimize radiation exposure.
The latest images from the nuclear plant showed severe damage to some of the buildings after the four explosions. Two of the buildings were a mangled mix of steel and concrete.
"The worst-case scenario doesn't bear mentioning and the best-case scenario keeps getting worse," Perpetual Investments said in a note on the crisis.
Financial leaders of the world's richest nations will hold talks on Friday on ways to calm global markets roiled by the crisis and concern it will unravel a fragile global economic recovery.
One G7 central banker, who asked not to be named, said he was "extremely worried" about the wider effects of the disaster in Japan, the world's third-largest economy.
Russian President Dmitry Medvedev, whose country is not part of the G7, called the situation a "colossal national disaster."