Japan's nuclear crisis deepened last night as engineers battled to prevent a meltdown in what is now the second-worst nuclear accident in history.
All non-essential staff were evacuated from the Fukushima Daiichi power plant last night after it suffered its third explosion in three days.
A cloud of radioactive dust billowed from the plant and radiation levels in the vicinity increased four-fold after the blast ripped through the No 2 reactor.
Government officials admitted it was "highly likely" the fuel rods in three separate reactors had started to melt despite repeated efforts to cool them with seawater.
Safety officials said they could not rule out a full meltdown as workers struggled to keep temperatures under control in the cores of the reactors.
The Fukushima crisis now rates as a more serious accident than the partial meltdown at Three Mile Island in the US in 1979, and is second only to the 1986 Chernobyl disaster, according to the French nuclear safety authority.
After insisting for three days that the situation was under control, Japan urgently appealed to US and UN nuclear experts for technical help in preventing white-hot fuel rods melting.
The International Atomic Energy Agency (IAEA) said it was "unlikely" the accident would turn into another Chernobyl.
More than 500,000 people are now thought to have been made homeless by Friday's earthquake and tsunami, which is estimated to have killed at least 10,000. More than 2,000 bodies have been washed up on beaches along Japan's Pacific coast, but rescuers have yet to reach isolated towns and villages in some of the worst-affected areas.
The tragedy is expected to become the costliest natural disaster in history, with the repair bill likely to top €115bn.
The economic impact was already being felt around the world yesterday as a 6.2pc fall in the Nikkei share index triggered huge losses on stock markets elsewhere.
Fears of a meltdown at the Fukushima Daiichi power station, 150 miles north of Tokyo, grew significantly after yesterday's two explosions.
The first blast at its No 3 reactor injured 11 workers and released as much radiation in an hour as would normally be expected in six months. It exposed up to 160 people to high doses, and 22 people received treatment for radiation poisoning.
Like the explosion in the No 1 reactor on Saturday, the problem was caused by a build-up of hydrogen released from water surrounding the reactor as temperatures rose above 2,200C.
The aircraft carrier USS Ronald Reagan later detected increased radiation 100 miles off the coast of Japan, and weather forecasters predicted that wind direction would change overnight and blow the radioactive cloud inland. Scientists said it did not pose a health risk.
Seventeen US helicopter crewmen helping with the relief effort were exposed to levels equivalent to one month's normal background radiation, but were declared free of contamination after being scrubbed down.
As technicians tried to contain the temperatures inside all three reactors at the plant, there were warnings of a third explosion as fuel rods inside the No 2 reactor became fully exposed.
Workers managed to pump enough seawater into the reactor to cover the rods, but they became partially exposed before last night's explosion.
Ryohei Shiomi, an official at Japan's Nuclear and Industrial Safety Agency, said the fuel rods in all three reactors appeared to be melting.
In the event of a complete meltdown, where the uranium core melts through the outer containment shell, high levels of radiation would be released into the environment, causing a major risk to health.
Yukiya Amano, director general of the IAEA, said: "The possibility of the development of this accident into one like Chernobyl is very unlikely."
He added that in Fukushima "the design is different and the structure is different", making the reactors far safer.
But local residents remained distrustful of assurances from the government.
Kyoko Nambu, whose house was destroyed by the tsunami, said: "It's like a horror movie. Our house is gone and now they are telling us to stay indoors.
"We can see the damage to our houses, but radiation? We have no idea what is happening. I am so scared."