There has been a fresh explosion at the earthquake damaged Japanese nuclear power plant of Fukushima.
Engineers had been pumping sea water into a reactor to prevent a meltdown in the wake of Friday’s earthquake and tsunami.
Non-essential staff are being removed from the Fukushima nuclear complex.
Radiation levels in the air surrounding the damaged plant rose four-fold after the explosion, the plant operator said.
The radiation reading at 8.31am Tuesday local time (11.31pm Monday Irish time) climbed to 8,217 microsieverts/hr from 1,941 about 40 minutes earlier, Tokyo Electric Power Co said.
Japanese authorities say levels would need to reach 1 million or so before causing large-scale radiation sickness.
Japan's nuclear safety agency said Tuesday's explosion at the plant's No 2 reactor was caused by hydrogen.
The complex has now seen explosions at three of its reactors on Saturday, Monday and Tuesday.
On Saturday, an explosion blew apart the building surrounding the plant's No 1 reactor but the seal around the reactor itself remained intact, officials said.
On Monday, a blast at its No 3 reactor shook the facility, injuring 11 people and sending plumes of smoke billowing into the sky.
A Tokyo Electric Power Co (TEPCO) spokesman said Tuesday's explosion, which happened between 6am and 6.15am Tuesday (9pm - 9.15pm Monday Irish time), was 'huge'.
The government also reported apparent damage to part of the container shielding the same reactor at Fukushima, although it was unclear whether this resulted from the blast.
Chief Cabinet Secretary Yukio Edano told reporters the suppression pool of the No 2 nuclear reactor appeared to have been damaged.
This is the bottom part of the container, which holds water used to cool it down and control air pressure inside.
A rapid drop in water levels exposed fuel rods in reactor No 2 at the Fukushima nuclear plant on Monday.
Japan has asked the United States for more equipment to help cool reactors at the Fukushima nuclear complex.
The UN's nuclear watchdog, the International Atomic Energy Agency, said Tokyo had asked for expert assistance.
However, the IAEA said on Monday that the crisis was unlikely to turn into another Chernobyl.
The fear at the Fukushima complex, 240km north of Tokyo, is of a major radiation leak after the events of Friday knocked out cooling systems.
US warships and planes helping with relief efforts temporarily moved away from the coast on Monday as a precaution because of low-level radiation.
South Korea, Hong Kong, Singapore and the Philippines said they would test Japanese food imports for radiation.
Death toll after Friday's quake / tsunami
Rescue workers combed the tsunami-battered region north of Tokyo, where officials say at least 10,000 people were killed in Friday's 9.0-magnitude earthquake and tsunami that followed it.
Tens of thousands of people are missing.
Prime Minister Naoto Kan said Japan was facing its worst crisis since World War Two.
About 850,000 households in the north are still without electricity in near-freezing weather, Tohuku Electric Power Co said.
The government said at least 1.5m households lack running water.
'It's a scene from hell, absolutely nightmarish,’ said Patrick Fuller of the International Red Cross Federation from the northeastern coastal town of Otsuchi.
'The situation here is just beyond belief, almost everything has been flattened,' he said of Otsuchi, a town all but obliterated.
'The government is saying that 9,500 people, more than half of the population, could have died and I do fear the worst.'
Kyodo news agency reported that 2,000 bodies had been found on Monday in two coastal towns alone.
Whole villages and towns have been wiped off the map by Friday's wall of water, triggering an international humanitarian effort of epic proportions.
Economic impact
Estimates of the economic impact are only now starting to emerge.
With the financial costs estimated at up to $180bn, analysts said it could tip the world's third biggest economy back into recession.
Hiromichi Shirakawa, chief economist for Japan at Credit Suisse, said in a note to clients that the economic loss will likely be around 14-15 trillion yen ($171-183bn) just to the region hit by the quake and tsunami.
Japanese stocks closed down more than 7.5% Monday, wiping $287bn off market capitalisation in the biggest fall since the height of the global financial crisis in 2008.
Toyota Corp dropped almost 8%.
The earthquake has forced many firms to suspend production.
Global companies from semiconductor makers to shipbuilders faced disruptions to operations after the quake and tsunami destroyed vital infrastructure, damaged ports and knocked out factories.
'The earthquake could have great implications on the global economic front,' said Andre Bakhos, director of market analytics at Lec Securities in New York.
'If you shut down Japan, there could be a global recession.'
Ireland 'a friend of Japan'
President Mary McAleese has conveyed her sympathies to Emperor Akihito of Japan on behalf of Ireland.
'As a friend of Japan, Ireland is steadfast in its solidarity with you and your people at this most difficult time,' Mrs McAleese said.
The Irish embassy in Tokyo has been in touch with all Irish citizens living in the affected areas.
Meanwhile, Tánaiste and Minister for Foreign Affairs Eamon Gilmore has offered Government support to the Red Cross relief operation.
Mr Gilmore said that while Japan was probably the best prepared country in the world for a major disaster, the crisis following the earthquake and tsunami was of an unprecedented magnitude.
The earthquake was the fifth most powerful to hit the world in the past century.
