About 200,000 people have been evacuated from residential areas around the Fukushima No.
1 plant, 250 kilometres (120 miles) north of Tokyo, as global concern grew over the threat of a major disaster.
International nuclear experts warned of the possibility of major accidents similar to the 1986 Chernobyl disaster in Ukraine and the 1979 partial core meltdown of the US Three Mile Island reactor.
Japan's top government spokesman said it was highly likely a meltdown had occurred in one reactor of the Fukushima plant and that it was working on the assumption that one may occur in another.
Chief Cabinet Secretary Yukio Edano added that the radiation released into the air so far had not reached levels high enough to affect human health.
The crisis started with Friday's massive quake and tsunami disaster, the worst on record in Japan, which caused electricity blackouts and led to malfunctions of the cooling systems of the plant's reactors.
Authorities immediately declared a nucl