Japan says it is stepping up efforts to cool reactors at the stricken Fukushima nuclear plant after army helicopters dumped tonnes of water on reactors to try to prevent a meltdown of fuel rods.
Four loads were dumped before the aircraft were forced to leave the site in order to minimise the crews' exposure to radiation. Officials say water cannon also joined the operation and they hope to have electricity restored soon.
The Japanese government says experts are investigating the effectiveness of the operation.
An earlier attempt to drop water from helicopters to cool the reactor was aborted because of fears about radiation levels above the plant.
The United States Nuclear Regulatory Commission has warned of the rising risk of a catastrophic radiation leak from spent fuel rods. Gregory Jaczko told a hearing in Washington that one reactor cooling pool for spent fuel rods may have run dry and another is leaking.
Mr Jaczko says the commission believes there are high levels of radiation around the reactor site, making it too dangerous for workers to get near. He says it is not accurate to say things are out of control at the plant, 250 kilometres northeast of Tokyo, the situation is nevertheless very serious.
One expert says there is now even a possibility the disaster may approach the extent of the Chernobyl accident, the worst in the nuclear industry's history.
There is growing alarm in the United States and elsewhere around the world about the crisis. A top US nuclear official says attempts to cool damaged reactors with sea water to prevent a meltdown appear to be failing and workers could be exposed to potentially lethal doses of radiation.
Safety zone
There is a 20-kilometre exclusion zone around the plant, though the United States Defence Department says it is not permitting American forces in Japan to go within 80 kilometres of the plant.
The Pentagon says soldiers have started prescribed medication amid growing concerns about radiation, though adding that the nuclear crisis will not stop its massive relief mission. Fourteen warships are taking up position off shore to ferry food and water to survivors.
The US navy has advised families at two bases to limit outdoor activities and shut off external ventilation after detecting higher-than-normal - but still low - doses of radiation.
Radiation fears
US Energy Secretary Steven Chu says the situation appears to be more serious than the partial meltdown at the Three Mile Island nuclear plant in Pennsylvania in 1979.
Engineers are working round the clock to avert catastrophe at the plant, which is leaking radiation into the atmosphere. There have been four explosions and two fires there since Saturday.
The plant, severely damaged by last week's earthquake and tsunami, has also been hit by a series of explosions in the days since.
Workers have struggled to top up water levels in the pools as the spent rods have heated up the water, threatening to evaporate it and expose the rods to air, sending out radioactive material.
Broadcaster NHK said earlier that police have been asked to send water cannon to cool the pool at the number four reactor.
Experts say the pools could present a bigger threat to public health than the reactors, which appear to be still encased in steel containment systems.
International Atomic Energy Agency head Yukiya Amano, a Japanese national, says damage to the core of three units at the plant has been confirmed.
One nuclear engineer says it may be time to consider ways of burying or covering the entire complex in some kind of material that would stop radiation from leaking into the atmosphere.
Financial meltdown
A stream of gloomy warnings and reports on the Japan crisis from experts and officials around the world has triggered a slump in global financial markets.
Japan's Nikkei average slumped on opening on Thursday, and an hour after opening was down nearly four per cent.
G7 Finance ministers will hold a conference call later in the day to discuss what can be done to help Japan cope with the financial and economic impact of the disaster.
The earthquake and tsunami disaster, and subsequent nuclear crisis, have wiped hundreds of billions of dollars off global stock markets.
