Japanese officials fear a meltdown at a nuclear power plant hit by Friday’s earthquake after radioactive material was detected outside it.
Japan’s nuclear agency said this meant fuel from one of the reactor’s cores may have started melting.
Japanese media reported an explosion and smoke at one of the Fukushima plants.
A huge relief operation is under way after the 8.9-magnitude earthquake and tsunami, which killed more than 400.
Another 784 people are missing.
Japan’s Prime Minister Naoto Kan declared a state of emergency at the Fukushima 1 and 2 power plants as engineers try to confirm whether a reactor at one of the stations has gone into meltdown.
Cooling systems inside several reactors at the plants stopped working after Friday’s earthquake cut the power supply.
Japan’s nuclear agency said on Saturday that radioactive caesium and iodine had been detected near the number one reactor of the Fukushima 1 plant.
The agency said this may indicate that containers of uranium fuel inside the reactor may have begun melting.
Air has been released from several of the reactors at both plants in an effort to relieve the huge amount of pressure building up inside.
Mr Kan said the amount of radiation released was “tiny”.
Thousands of people have been ordered to evacuate the area near the plants.