By Chisa Fujioka and Jon Herskovitz
TOKYO (Reuters) - 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 parliamentary 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."
But Japanese Economics Minister Kaoru Yosano told Reuters the country's markets were not unstable enough to warrant joint G7 currency intervention or government purchases of shares.
The yen surged to a record high against the dollar on market speculation Japan would repatriate funds to pay for the massive cost of post-disaster reconstruction. The yen rose as high as 76.25 per dollar, surpassing the previous record high of 79.75 reached in the wake of the Kobe earthquake in 1995.
Japan's Nikkei average fell sharply on opening on Thursday, but ended the day down just 1.44 percent. The Nikkei has fallen more than 12 percent this week.
CRITICAL REACTOR CONTAINS PLUTONIUM
High radiation levels on Wednesday prevented helicopters from dropping water into reactor No. 3 to try to cool its fuel rods after an earlier blast damaged its roof and cooling system.
Another attempt on Thursday appeared to partly succeed, with two of four water drops over the site hitting their mark. The giant, twin-blade aircraft have to make precisely timed flyovers and drops to avoid the brunt of the radiation.
The plant operator described No. 3 -- the only reactor that uses plutonium in its fuel mix -- as the "priority." Experts described plutonium as a pernicious isotope that could cause cancer if very small quantities were ingested.
Sebastian Pflugbeil, president of the private German-based Society for Radiation Protection, said Japan's efforts to pull the Fukushima plant back from the brink signaled "the beginning of the catastrophic phase."
"Maybe we have to pray," he said, adding that a wind blowing any nuclear fallout east into the Pacific would limit any damage for Japan's 127 million people in case of a meltdown or other releases, for instance from spent fuel storage pools.
Low and harmless concentrations of radioactive particles were heading from Japan toward the United States, Lars-Erik De Geer, research director at the Swedish Defense Research Institute, a government agency, said, citing data from a network of international monitoring stations.
The government warned Tokyo's 13 million people to prepare for a possible large-scale blackout but later said there was no need for one. Still, many firms voluntarily reduced power, submerging parts of the usually neon-lit city in darkness.
In a possible sign of panic, one bank, Mizuho, said all its automated teller machines in the country crashed twice in the day after excessive transactions at some branches.
ANXIETY IN TOKYO - AIRPORTS BUSY, STREETS QUIET
A U.S. State Department official said flights would be laid on for Americans to leave Japan, and family of embassy staff had been authorized to go if they wanted.
Scores of flights to Japan have been halted or rerouted and air travelers are avoiding Tokyo for fear of radiation.
On Thursday, the U.S. embassy in Tokyo urged citizens living within 80 km (50 miles) of the Daiichi plant to evacuate or remain indoors "as a precaution," while Britain's foreign office urged citizens "to consider leaving the area."
The latest warnings were not as strong as those issued earlier by France and Australia, which urged nationals in Japan to leave the country. Russia said it planned to evacuate families of diplomats on Friday, and Hong Kong urged its citizens to leave Tokyo as soon as possible or head south.
Japan's government has told people within 30 km (18 miles) of the plant to stay indoors.
At its worst, radiation in Tokyo has reached 0.809 microsieverts per hour this week, 10 times below what a person would receive if exposed to a dental x-ray. On Thursday, radiation levels were barely above average.
But many Tokyo residents stayed indoors, usually busy streets were nearly deserted and many shops were closed. At the second-floor office of the Tokyo Passport Center in the city's Yurakucho district, queues snaked to the first floor.
"Since yesterday we have had one-and-a-half times more people than usual coming to apply for a passport or to enquire about getting one," said Shigeaki Ohashi, a passport official.
SNOW COMPOUNDS MISERY FOR TSUNAMI SURVIVORS
The plight of hundreds of thousands left homeless by the earthquake and tsunami worsened following a cold snap that brought heavy snow to worst-affected areas.
Supplies of water and heating oil are low at evacuation centers, where many survivors wait bundled in blankets.
About 850,000 households in the north were still without electricity in near-freezing weather, Tohuku Electric Power Co. said, and the government said at least 1.5 million households lack running water.
"It's cold today so many people have fallen ill, getting diarrhea and other symptoms," said Takanori Watanabe, a Red Cross doctor in Otsuchi, a low-lying town where more than half the 17,000 residents are still missing.
The National Police Agency said it has confirmed 4,314 deaths in 12 prefectures as of midnight Wednesday, while 8,606 people remained unaccounted for in six prefectures.
(Additional reporting by Linda Sieg, Terril Yue Jones, Nathan Layne, Elaine Lies, Leika Kihara and Mayumi Negishi; Writing by Nick Macfie and Jason Szep; Editing by John Chalmers and Dean Yates)