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"They are leaving us to die," says the mayor of Minamasoma inside the exclusion zone
Increasing alarm has been expressed in the US about the crisis at Fukushima Daiichi nuclear plant in Japan.
Greg Jaczko, chairman of the US Nuclear Regulatory Commission (NRC), said attempts to cool reactors with sea water and prevent them from melting down appeared to be failing.
Emergency workers in the vicinity could be exposed to "potentially lethal" radiation doses, he said.
The plant was severely damaged by last week's huge earthquake and tsunami.
US Energy Secretary Steven Chu has said that the situation at the plant appears to be more serious than the partial meltdown at the Three Mile Island nuclear plant in Pennsylvania in 1979.
Analysis
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.
The US state department has urged Americans living within 80km (50 miles) of Fukushima Daiichi, which lies 220km from Tokyo, to leave the area - a much wider exclusion zone than the 20km advised by the Japanese government.
Some US military personnel in Japan have been given tablets against possible radiation effects.
Britain has now advised its nationals currently in Tokyo and to the north of the capital to consider leaving the area.
Engineers are racing to avert a nuclear catastrophe at Fukushima Daiichi, where the tsunami wrecked back-up diesel generators that kept the nuclear fuel cool.
Workers have been dousing the reactors with sea water in a frantic effort to stabilise their temperatures, since the first in a series of explosions rocked the plant on Saturday.
But helicopters deployed to dump water on the facility on Wednesday were pulled out amid concerns over radiation levels in the air above the site.
Police units have been asked to bring in water cannons, normally used in riot control, to spray water on the fuel.
The power facility has sent low levels of radiation wafting into Tokyo.
Plant operator Tokyo Electric Power Co. has said it is seeking to restore the power supply to the plant's cooling systems "as soon as possible".
The US NRC has 11 agency experts in Tokyo monitoring the situation.
Mr Jaczko told a congressional energy and commerce subcommittee hearing in Washington that there appeared to be serious problems with attempts to cool three reactors.
"We believe that around the reactor site there are high levels of radiation," he said.
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 on Tuesday and another fire on Wednesday
"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."
The head of the UN's atomic energy agency, Yukio Amano, is travelling to Japan in person to gather more information.
Earlier, 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".
The 77-year-old - deeply respected by many Japanese - said: "I hope from the bottom of my heart that the people will, hand in hand, treat each other with compassion and overcome these difficult times."
Fukushima prefecture governor Yuhei Sato has criticised official handling of the evacuation of the area around the stricken power plant. "Anxiety and anger felt by people have reached boiling point," he said.
Mr Sato said centres already housing people who had been moved from their homes near the plant did not have enough hot meals and basic necessities such as fuel and medical supplies. "We're lacking everything," he said.
In other developments:
- After losing $620bn (£385bn) in the first two days of this week, Japan's stock market rebounded to finish Wednesday up by 5.7%
- France urged its nationals in Tokyo to leave the country or move south; two Air France planes were sent to begin evacuation
- Australia advised its citizens to consider leaving Tokyo and the most damaged prefectures
- Turkey warned against travel to Japan
Thousands of people were killed in the 9.0-magnitude quake and tsunami.
Snow has blanketed swathes of the disaster zone, where many survivors have little food, water or heat.
About 450,000 people have been staying in temporary shelters, many sleeping on the floor of school gymnasiums.
More than 4,300 people are listed as dead but it is feared the total death toll from the catastrophe, which pulverised the country's north-east coast, will rise substantially.
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