There is a risk of a second explosion at the quake-hit Fukushima power station, Japanese officials have said.
However, chief cabinet secretary Yukio Edano said the facility could withstand the impact and the nuclear reactor itself would not be damaged.
Technicians are battling to cool reactor 3 following a blast at the building housing reactor 1 on Saturday.
Meanwhile, police have warned that the death toll in tsunami-hit Miyagi prefecture alone could exceed 10,000.
Miyagi includes the port of Minamisanriku which was mostly swept away by the tsunami.
The BBC's Rachel Harvey, outside Minamisanriku, says roads to the town are blocked and emergency workers are finding bodies amid piles of debris.
Officials previously said that more than 2,000 people died or were missing following Friday's earthquake and tsunami.
Millions remain without electricity and authorities are stepping up relief efforts as the scale of the tragedy becomes clearer.
About 310,000 people have been evacuated to emergency shelters, many of them without power, Japanese broadcaster NHK reported.
Officials announced that the number of troops helping with rescue work in the region would be doubled to 100,000.
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