TOKYO - Operators of a quake-crippled nuclear plant in Japan dumped water on overheating reactors on Thursday while the United States expressed growing alarm about leaking radiation and said it was sending aircraft to help Americans leave the country.

Engineers were rushing against time to run power from the main grid to fire up water pumps needed to cool two reactors and the spent fuel rods considered to pose the biggest risk of spewing radioactivity into the atmosphere.

While Japanese officials were scrambling with a patchwork of fixes, the top US nuclear regulator warned that 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 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.

The plant operator said it believed the No 4 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 tons of water, all aimed at this reactor.

US officials took pains not to criticise the Japanese government, which has shown signs of being overwhelmed by the crisis, but Washington's actions indicated a divide with the Japanese about the perilousness of the situation.

"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.

Japan said the United States would fly a high-altitude drone over the stricken complex to gauge the situation, and had offered to send nuclear experts.

A State Department official said flights would be laid on for Americans to leave and family of embassy staff had been authorised to leave if they wanted.

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 9.0 magnitude earthquake and tsunami, such as cold, heavy snow in parts and access to fresh water.  

The latest images from the nuclear plant showed severe damage to some of the buildings after several explosions.

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 signalled "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.

A stream of gloomy warnings and reports on the Japan crisis from experts and officials around the world triggered a sharp fall in US financial markets, with all three major stock indexes slumping on fears of slower worldwide growth.

Economics Minister Kaoru Yosano told Reuters that Japan's markets were not unstable enough to warrant joint G7 currency intervention or government purchases of shares.

"I don't think stock and currency markets are in a state of turmoil," Yosano said.

Earlier, one G7 central banker, who asked not to be named, said he was "extremely worried" about the wider effects of the crisis in Japan, the world's third-largest economy.

G7 finance ministers will hold a conference call later on Thursday to discuss steps to help Japan cope with the financial and economic impact of the disaster, a source said.  

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.  

Japanese Finance Minister Yoshihiko Noda blamed speculation for the yen's surge.

Japan's Nikkei average fell sharply on opening on Thursday, and was down about 1.3 percent at 0555 GMT.

 

BID TO GET POWER RESTORED TO REACTORS

Japan's nuclear agency said radiation levels at the plant "continued to fall", but the government appealed to private companies to help deliver supplies to tens of thousands of people evacuated from a 30-km (18-mile) zone around the complex.

Tokyo Electric Power Co. (TEPCO) officials said bulldozers attempted to clear a route to the reactor so fire trucks could gain access and try to cool the facility using hoses.  

High radiation levels on Wednesday prevented a helicopter from dropping water into reactor No 3 to try to cool its fuel rods after an earlier explosion 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 at that uses plutonium in its fuel mix -- as the "priority".   Experts described plutonium as a very nasty isotope that could cause cancer if very small quantities were ingested.  

The situation at No 4 reactor, where the fire broke out, was "not so good", TEPCO said on Wednesday, while water was being poured into reactors No 5 and 6, indicating the entire six-reactor facility was now at risk of overheating.

Inside the wrecked complex, torn apart by at least four explosions, workers toiled in protective suits and makeshift lighting to try to monitor what was going on inside the reactors despite dangerously high radiation levels. They work in short shifts to minimise radiation exposure.

 

EMBASSIES URGE CITIZENS TO LEAVE

Scores of flights to Japan have been halted or rerouted and air travellers are avoiding Tokyo for fear of radiation.  

On Thursday, the US 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 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.

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. Early on Thursday, radiation levels were barely above average.  

But many Tokyo residents stayed indoors, usually busy streets were nearly deserted and many shops and offices were closed.  

One bank, Mizuho , said all its automatic teller machines in the country briefly crashed. It doubted the problem was connected to the earthquake or power cuts, but it triggered a rush to withdraw cash from machines in the capital.