The evacuation of residents near Japan's quake-hit Fukushima nuclear plant will be long-term, officials say.
Many of the tens of thousands of people evacuated from the area around the plant are living in temporary shelters.
The announcement came as high levels of radiation were detected for the first time in groundwater near one of the facility's six reactors.
Meanwhile, a massive search has begun to find the remains of those missing since the devastating tsunami hit.
Three weeks after the 9.0-magnitude earthquake and subsequent tsunami, the true number of those who died is still not known.
More than 11,500 people are confirmed dead but nearly 16,500 remain unaccounted for.
More than 100 Japanese and US military planes and 65 ships are scouring the country's north-eastern coast to locate any remaining bodies.
Employing some 24,000 military personnel, the three-day air and sea operation will focus on shores that were largely submerged or remain under water, as well as the mouths of major rivers.
Many coastal areas remain inaccessible to rescuers trying enter by road or foot, blocked by the mangled remains of houses, ships, cars and trains.
Because of radiation concerns, the search does not include the 20km (12-mile) evacuation zone around the Fukushima Daiichi plant, where there are believed to be 1,000 bodies.
More than 70,000 people living within the exclusion zone have been moved to temporary shelters.
Another 136,000 people who live within 20-30km of the plant have been encouraged by the authorities to leave or to stay indoors.
Chief Cabinet Secretary Yukio Edano said the evacuation would be "long-term".
In a televised address, Prime Minister Naoto Kan said he was "prepared for a long-term battle" at the plant - one he said that would be won.
"At the current stage, we cannot say that the plant has been sufficiently stabilised. But we are preparing for all kinds of situations and I am convinced that the plant can be stabilised.
"We cannot say at this stage say by when this will happen, but we are trying our best," he said.
The authorities are resisting calls from the UN's atomic agency to expand the exclusion zone around the plant, after it found safe radiation limits had been exceeded at the village of Iitate, 40km away.
Highly radioactive water continues to leak at the plant; for the first time it has been found in groundwater 15m below reactor 1.
Although it does not appear to have caused an immediate problem, there is a possibility it could eventually affect drinking water if concentrations were high enough.
Radioactive material detected in the sea near the plant rose steeply on Thursday, with radioactive iodine levels reaching 4,385 times the limit.
In the latest twist, the much-criticised operator of the plant, Tepco, has said it will review all data on radiation leaked from the plant, citing errors in a computer program.
Among the measurements called in to question, was one that showed groundwater containing radioactive iodine concentrations of 10,000 times the government standard, the nuclear safety agency said.
Seawater and air concentrations from this week also are under review.
Workers are continuing to try to stabilise four reactors by using water to cool fuel rods. They also face the problem of how to deal with highly radioactive run-off water that has accumulated in a tunnel.
Tepco is also under fire after it emerged there was a shortage of radiation monitors for workers.
Many of the monitors were destroyed in the tsunami, so Tepco had assigned one per group of workers rather than per individual, Japan's nuclear safety agency said.
"The agency warned Tepco yesterday (Thursday) to do the utmost to manage workers' exposure levels," said spokesman Hidehiko Nishiyama.
Radioactive material may be leaking from the damaged plant continuously, the country's nuclear and industrial safety agency (Nisa) said.
The radiation, which has halted shipments of certain vegetables, dairy produce and other foodstuffs from four nearby prefectures, has been widened to include beef, the health ministry said.
However, the government insists that no water or food contamination has reached levels that would be harmful to people's health.