Japan says it is stepping up efforts to cool reactors at the tsunami-stricken Fukushima Daiichi nuclear plant.
Army helicopters dumped tonnes of water to try to prevent a meltdown of fuel rods. Water cannon will join the operation shortly and it is hoped electricity will be restored soon.Increasing alarm has been expressed in the US at the crisis.
The confirmed death toll from Friday's 9.0 magnitude quake, which triggered a tsunami, has risen above 5,000.
Police say 5,178 are confirmed dead and another 8,606 people are still missing.
'Deep condolences' Japan's military CH-47 Chinook helicopters began spraying tonnes of water on reactors three and four at Fukushima, 220km (137 miles) from Tokyo, at 0948 local time (0048 GMT), officials said.
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Over the days of the Fukushima crisis, attention has switched from reactor building 1 to 3, to 2, back to 3 - and now, to 4.
Here, it is not the actual reactor that is causing concern. Instead, it is a pool storing fuel rods that had been taken out of the reactor when it was shut down for maintenance before the earthquake struck.
There have been reports that water levels were low; and now the US Nuclear Regulatory Commission (NRC), which has a team of experts advising in Japan, says the pool is completely dry.
This means the fuel rods are exposed to the air. Without water, they will get much hotter, allowing radioactive material to escape - and the NRC says radiation levels are probably extremely high, creating a danger to workers at the plant.
The company operating the plant has even warned of "re-criticality" - that a nuclear chain reaction could start among fuel rods in the now dry pool.
That would not cause a nuclear explosion but it would increase the release of radioactive substances.
Analysis
Here, it is not the actual reactor that is causing concern. Instead, it is a pool storing fuel rods that had been taken out of the reactor when it was shut down for maintenance before the earthquake struck.
There have been reports that water levels were low; and now the US Nuclear Regulatory Commission (NRC), which has a team of experts advising in Japan, says the pool is completely dry.
This means the fuel rods are exposed to the air. Without water, they will get much hotter, allowing radioactive material to escape - and the NRC says radiation levels are probably extremely high, creating a danger to workers at the plant.
The company operating the plant has even warned of "re-criticality" - that a nuclear chain reaction could start among fuel rods in the now dry pool.
That would not cause a nuclear explosion but it would increase the release of radioactive substances.
The aircraft dumped four loads before leaving the site in order to minimise the crews' exposure to radiation. On Wednesday, the helicopters were forced to abort a similar operation amid concerns over high radiation levels.
The BBC's Chris Hogg in Tokyo says the helicopters can carry an enormous amount of water but given the high winds it is difficult to know whether it has been dropped accurately.Japanese government spokesman Yukio Edano said at a news briefing that nuclear experts were now investigating how effective the operation was.
Meanwhile, water trucks are on standby to spray more water on the reactors.
The operation was intended to help cool the reactors and also to replenish water in a storage pond with spent fuel rods.
Officials also said they were hoping that later on Thursday they would restore the power supply to the plant, which is needed for the cooling system and backup generators.
"If the restoration work is completed, we will be able to activate various electric pumps and pour water into reactors and pools for spent nuclear fuel," a spokesman for Tokyo Electric Power Company, which runs the plant, told the AFP news agency.
US President Barack Obama spoke to Prime Minister Naoto Kan on Thursday local time amid increasing US fears over the crisis.
The US has now chartered aircraft to help Americans leave Japan and authorised the voluntary departure of relatives of diplomatic staff.
The White House said Mr Obama conveyed "deep condolences" at the loss of life and said Washington was "determined to do everything possible to support Japan".
On Wednesday, Greg Jaczko, chairman of the US Nuclear Regulatory Commission (NRC), told a congressional energy and commerce subcommittee hearing in Washington that there appeared to be serious problems with attempts to cool the reactors.
"We believe that around the reactor site there are high levels of radiation."
He said 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".
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Fukushima Daiichi: What went wrong
- Reactor 1: Was first to be rocked an explosion on Saturday; fuel rods reportedly 70% damaged
- Reactor 2: There are fears a blast on Tuesday breached a containment system; fuel rods reportedly 33% damaged
- Reactor 3: Explosion on Monday; smoke or steam seen rising on Wednesday; damage to roof and possibly also to a containment system
- Reactor 4: Hit by a major blaze (possible blast) on Tuesday and another fire on Wednesday
US Energy Secretary Steven Chu said the situation appeared to be more serious than the partial meltdown at the Three Mile Island nuclear plant in Pennsylvania in 1979.
The head of the UN's atomic energy agency, Yukio Amano, is travelling to Japan in person to gather more information.The US state department has urged Americans living within 80km of Fukushima Daiichi to leave the area - a much wider exclusion zone than the 20km advised by the Japanese government.
Mr Edano described the US approach as "conservative".
Britain has advised its nationals currently in Tokyo and to the north of the capital to consider leaving the area.
France has urged its citizens in Tokyo to leave the country or move south. Two Air France planes are due to begin evacuating French nationals later on Thursday.
In areas of the north-east badly hit by the tsunami, bitter winter weather has added to the misery of survivors, though more supplies are now reported to be reaching them.
The governor of Fukushima prefecture, where the badly damaged nuclear plant is located, has complained that evacuation centres lack basic necessities, including sufficient hot food.
About 450,000 people have been staying in temporary shelters, many sleeping on the floor of school gymnasiums.
The crisis has also continued to affect the markets - the benchmark Nikkei index fell 3.6% in early Thursday trading in Tokyo, shortly after the yen briefly hit the highest level against the US dollar since World War II.
On Wednesday, in a rare public appearance, Japan's Emperor Akihito said he was "deeply worried" about the crisis his country was facing.
TV stations interrupted programming to show the emperor describing the crisis facing the nation as "unprecedented in scale".
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