One of the four buildings at the damaged Fukushima I nuclear plant has been destroyed in an apparent explosion, Japan's NHK broadcaster reported Saturday.
Witnesses heard the sound of an explosion and saw white smoke emerging from the plant, NHK reported. The cause was not known.
Tokyo Electric Power Co, the plant's operator, said four people had been injured, according to the Kyodo news agency.
Technicians had been working at releasing pressure from the plant's reactors to avert a meltdown of the reactors.
Increased levels of radiation had been detected in the area of the power plant following Friday's earthquake.
Radioactive caesium has been detected in the vicinity of the damaged nuclear plant in north-eastern Japan, Kyodo news reported, quoting the Nuclear and Industrial Safety Agency.
The presence of the substance is an indication of a meltdown.
Earlier Saturday, authorities extended the evacuation zone to residents living within 10 km of the Fukushima Daiichi plant, also known as Fukushima I, where the cooling system experienced troubles Friday.
Officials at a checkpoint 60 km from the plant warned of danger in the area, according to the BBC.
Meanwhile, residents within 3 km of the Fukushima Daini power plant, known as Fukushima II, were ordered to leave their homes.
The government was holding a crisis meeting Saturday to discuss the situation at the two nuclear power plants. Authorities planned to release radioactive steam in order to relieve pressure on the reactors and prevent a disaster.
Radiation measurements inside the Fukushima Daiichi plant were 1,000 times higher than normal after the massive earthquake, Kyodo news reported earlier.
Authorities were concerned that radioactivity may have escaped the plant due to high pressure inside an overheating reactor. The earthquake damaged power supplies and disrupted the reactor's cooling systems. An observation post near the plant's gate recorded radiation levels eight times higher than normal.
The cooling system for three reactors at Fukushima Daini were also not operating, Kyodo news reported.
