Japanese military helicopters and fire trucks poured water on an overheating nuclear facility today and the plant operator said electricity to part of the crippled complex might be restored in a bid to avert catastrophe.
Washington and other foreign capitals have expressed growing alarm about radiation leaking from the earthquake-shattered plant, 240 km north of Tokyo. The US said it was sending aircraft to help Americans leave Japan.
The International Atomic Energy Agency (IAEA) said the situation at the damaged plant remained very serious but had not worsened in the last 24 hours.
Graham Andrew, a senior IAEA official said the plant's No4 reactor was a "major safety concern." He told a news conference: "The current situation at the Fukushima Daiichi plant remains very serious ... (but) there has been no significant worsening since yesterday."
Workers were trying to connect a 1km-long power cable from the main grid to restart water pumps to cool reactor No 2, which does not house spent fuel rods considered the biggest risk of spewing radioactivity into the atmosphere.
One official from the plant operator said the cable could be connected within hours.
Other officials said it was unclear if water pumps at reactor No 2, which sustained less damage from a series of explosions, would work.
While US officials were at pains not to criticise Japan's government, Washington's actions hinted at a divide with its ally about its concern over the potential for the world's worst nuclear accident since the Chernobyl disaster in 1986.
The top US 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 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 yesterday, and made clear its priority was the spent-fuel pool at the No 3 reactor.
This morning 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.
High radiation levels yesterday 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.
The helicopters 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".
Health experts said panic over radiation leaks from the plant 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 massive 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 minimise radiation exposure.
The latest images from the nuclear plant showed severe damage after the blasts. Two of the buildings were a mangled mix of steel and concrete.
Financial leaders of the world's richest nations will hold talks tomorrow 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 called the situation a "colossal national disaster" and has offered to send experts to assist with the crisis.
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 government warned Tokyo's 13 million people to prepare for a possible large-scale blackout but later said there was no need for one.
A US State Department official said flights would be laid on for Americans to leave Japan, and family of embassy staff had been authorised to go if they wanted.
Scores of flights to Japan have been halted or rerouted and air travellers are avoiding Tokyo for fear of radiation.
The US embassy in Tokyo has urged citizens living within 80km 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 Department of Foreign Affairs is advising Irish citizens in the northeast of Japan or in Tokyo to leave as soon as possible, particularly if they have small children. France and Australia have also urged their nationals in Japan to leave the country.
Russia said it planned to evacuate families of diplomats tomorrow, 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 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.
Today, radiation levels were barely above average.
But many Tokyo residents stayed indoors, usually busy streets were nearly deserted and many shops were closed.
Reuters