There is no water left in the spent fuel pool of reactor 4 at Japan’s Fukushima nuclear plant, resulting in “extremely high” radiation levels, the chair of the US Nuclear Regulatory Commission warned overnight.
As the nuclear emergency deepened and the death toll from the March 11 earthquake and tsunami passed 12,000, NRC chairman Gregory Jaczko told US legislators that if America was facing a similar situation, it would order a much larger evacuation zone than Japan has (20km), and so the US has called on Americans within 50 miles (80 kilometres) of the Japan nuclear plant area to leave.
Japanese military helicopters were due to dump water on the nuclear plant, which has been hit by four explosions and two fires, to help contain the overheating, but were forced back due to radiation.
Engineers have been desperately battling a feared meltdown at the 40-year-old plant since the earthquake and tsunami knocked out cooling systems and fuel rods began overheating.
But chief government spokesman Yukio Edano said radiation levels from the plant posed no immediate health threat outside a 20km exclusion zone that has already been evacuated.
However Mr Jaczko said he understood that secondary containment had been destroyed and there is no water in the spent fuel pool.
"We believe that radiation levels are extremely high which could possibly impact the ability to take corrective measures," he said.
But the head of a French nuclear safety agency said that while the Fukushima accident had generated a “radioactive plume” which was likely to expand in coming days, it did not present any health threat to Tokyo.
He added, though, that there could eventually be a “strongly contaminated zone” extending up to 60km around the stricken plant.
“In the coming days, this plume will eventually reach a zone of several hundred kilometres, but our calculations show that, for example in Tokyo, exposure will not have any impact on health in any way,” said Jacques Repussard, from the Institute for Radiological Protection and Nuclear Safety (IRSN).
The Fukushima No. 1 plant, hit by the quake, lies 250km north-east of the Japanese capital.
Japan's emperor gave a rare address to a jittery nation overnight as millions struggled in desperate conditions after last week's quake and tsunami disaster.
The television appearance by Emperor Akihito emphasised the gravity of the crisis gripping Japan after the 9.0-magnitude quake and the monster waves it unleashed, killing thousands and crippling a nuclear power plant.
Akihito said he was "deeply concerned" about the "unpredictable" situation at the stricken Fukushima No.1 power plant, which has been hit by a series of explosions after Friday's quake knocked out reactor cooling systems.
"I sincerely hope that we can keep the situation from getting worse," Akihito said, in a historic televised address that marked the first time he has intervened in a national crisis.
Japanese crews grappling with the world's worst nuclear incident since Chernobyl in 1986 briefly suspended work after a spike in radiation levels at the plant 250km north-east of Tokyo.
And already jangled nerves were frayed further by a series of aftershocks including a strong 6.0-magnitude earthquake that swayed buildings in Tokyo.
The official toll of the dead and missing after the quake and tsunami flattened Japan's northeast coast rose to more than 12,000, police said, with the number of confirmed dead at 4277.
But reports continued to come in which indicated that the final grisly toll could be much higher, with the mayor of the coastal town of Ishinomaki saying the number of missing there was likely to hit 10,000, Kyodo News reported.
On Saturday, public broadcaster NHK reported that around 10,000 people were also unaccounted for in the port town of Minamisanriku, again in Miyagi prefecture.
After the Tokyo stock exchange's biggest two-day sell-off in 24 years sparked a global market rout, the headline Nikkei share index closed up 5.68 per cent on bargain hunting.
The Bank of Japan pumped another Y3.5 trillion ($A43.81 billion) into the financial system, adding to trillions spent this week since the disaster crippled a large swath of the economy.
The evacuation order at the Fukushima nuclear power plant came as a tall white cloud was seen billowing into the sky over the stricken complex.
Earlier, crews at Fukushima contended with a new fire and feared damage to the vessel containing one of the plant's six reactor cores.
The 50 or so workers at the plant have been hailed as heroes.
"Please don't forget that there are people who are working to protect everyone's lives in exchange for their own lives," said one post on Japanese social networking site Mixi.
Aside from the nuclear threat, the full scale of the quake and tsunami disaster was becoming clear as more details emerged of the staggering death and devastation in the worst-hit northeast.
"The number of people killed is increasing day by day and we do not know how many people have fallen victim," said the emperor, who is held in deep respect by many Japanese. "I pray for the safety of as many people as possible."
"People are being forced to evacuate in such severe conditions of bitter cold, with shortages of water and fuel.... I cannot help praying that rescue work is done swiftly and people's lives get better, even a little."
Millions have been left without water, electricity, fuel or enough food and hundreds of thousands more are homeless, stoically coping with freezing cold and wet conditions in the northeast.
Aomori governor Shingo Mimura said he desperately needed central government assistance to get hold of oil and relief supplies.
"We cannot possibly get out to rescue survivors nor reconstruct the devastated areas without oil," he said.
"There are a variety of problems, such as shortages of water, food and blankets as well as difficulties in delivering supplies," added Ryu Matsumoto, state minister in charge of disaster management.
Tokyo Electric Power Co said three-hour power outages on Wednesday would affect 10.89 million households.
The governor of Fukushima prefecture, home to the crippled nuclear plant, said people were at breaking point.
"The worry and anger of the people of Fukushima has been pushed to the limit," Yuhei Sato told NHK.
With nerves on edge across the world's third-biggest economy and beyond, people across Asia have been stripping shelves of essentials for fear of a major emission of radiation from the power plant on the east coast.
The Japanese government has warned that panic buying in towns and cities that have not been directly affected by the twin disasters could hurt its ability to provide aid to the devastated areas.
The normally heaving streets and subways of Tokyo were quieter than usual on Wednesday morning. The number of people sporting paper face masks has shot up, although the masks offer no real protection against radiation.
Radiation levels in the capital's vast urban sprawl of 30 million people have see-sawed without ever reaching harmful levels, according to the government.
But it has warned people living up to 10km beyond the 20km exclusion zone around the Fukushima plant to stay indoors. More than 200,000 people have already been evacuated from the zone.