Pressure is mounting on Japan to expand the evacuation zone around the Fukushima Daiichi power plant, as the prime minister says he plans to review the country's nuclear energy policy.

The International Atomic Energy Agency (IAEA) said Japanese authorities should consider expanding the zone beyond its current 20km (12-mile) radius after high levels of radiation were detected at a village about twice that distance from the plant.

The government has so far resisted calls to evacuate more people from the area, but said its policy was under constant review, and that monitoring of radiation levels was being increased.

More than 70,000 people living inside the 20km zone have been evacuated, but another 136,000 living between 20-30km away have been told to stay in their homes. The US has recommended that its citizens stay at least 80km away.

Some have taken government advice to leave voluntarily, but many others have spent almost three weeks living in an area with few supplies and services, their plight compounded by rising radiation levels and speculation that stabilising Fukushima Daiichi could take months.

Radiation fears have prevented authorities from collecting the bodies of as many as 1,000 people living in the evacuation zone who died in the 11 March earthquake and tsunami.

Kyodo news agency cited police sources as saying the corpses had been exposed to high radiation levels and would probably have to be decontaminated before they could be collected and examined by doctors.

Left as they were, the bodies could pose a health threat to relatives identifying them at morgues, the agency said. Cremating them could create radioactive smoke, while burying them could contaminate soil.

The IAEA said measurements taken at Iitate, 40km from the plant, were above the level at which the United Nations body normally orders evacuations. Earlier this week, Greenpeace issued a similar warning after recording high levels of radiation in the village.

"We have advised [Japanese officials] to carefully assess the situation, and they have indicated that it is already under assessment," Denis Flory, a senior IAEA official, said in Vienna.

"The highest values were found in a relatively small area in the north-west from the Fukushima power plant and the first assessment indicates that one of the IAEA operational criteria for evacuation is exceeded in Iitate village."

The agency said its latest readings were conducted over a wide area from 18-26 March, and that the samples contained radioactive iodine-131 and caesium-137.

But the chief government spokesman, Yukio Edano, said the evacuation zone would stay unchanged for the time being. "At the moment, we have no reason to believe that the radiation will have an effect on people's health," he said. "We need to step up our monitoring, and if necessary take steps to deal with it."

Media reported that 140 members of a US military team specialising in radiation control would arrive soon to help deal with the crisis.

Nuclear safety officials said rising contamination in the sea near the plant pointed to a constant leak of radiation. On Thursday, Japan's nuclear and industrial safety agency said radioactive iodine near drains running from the plant was 4,385 times higher than the legal limit.

Experts said workers at the plant, 240km north of Tokyo, faced the problematic task of cooling overheating reactors with seawater while ensuring that contaminated runoff does not end up in the surrounding sea and soil.

"There's definitely a conflict now between trying to keep the reactors cool and managing the contaminated waste water being generated by the operation," said Ed Lyman, of the US-based Union of Concerned Scientists.

The deepening of the crisis exposed a dispute between the government and the plant's operator, Tokyo Electric Power company about the plant's future. The firm has said four of the six reactors are beyond repair, but that two could function again. The prime minister, Nato Kan, however, said the entire plant should be decommissioned.

Kyodo reported that Kan is to order a review of plans to increase Japan's dependence on nuclear energy from 30% to 50% by 2030. With public confidence in the industry severely dented by the Fukushima emergency, few communities are expected to grant approval for the construction of 14 atomic power plants over the next 20 years.