Tokyo, March 12 (DPA) An explosion occurred at the damaged Fukushima I nuclear plant, injuring four people, Japan's NHK broadcaster reported Saturday.

It appeared that the roof of one of the buildings housing a reactor had collapsed, NHK reported. A government spokesman described the situation as "potentially very serious", but called upon the populace to remain calm, reported NHK.

Technicians had been scrambling to reduce pressure in the reactors to prevent a meltdown, after the plant was damaged in Friday's 8.9-magnitude quake, Japans' nuclear safety agency said.

Radioactive caesium has been detected in the vicinity of the plant in northeastern Japan. The presence of the substance is an indication of a meltdown.

The plant's operator, Tokyo Electric Power Co (TEPCO), managed to depressurize the containers housing the reactors, the agency said, but that activity involved relasing steam that would likely contain radioactive materials.

Technicians encountered difficulties opening a pressure release valve due to high radioactive levels, the Kyodo News Agency quoted the Nuclear and Industrial Safety Agency as saying.

Earlier Saturday, authorities extended the evacuation zone to residents living within 10 kilometres of the Fukushima Daiichi plant, also known as Fukushima I, where the cooling system experienced troubles Friday.

Evacuation of residents had not been completed by early Saturday evening, Kyodo said.

Meanwhile, residents within three kilometres of the Fukushima Daini power plant, known as Fukushima II, were also ordered to leave their homes.

The government was holding a crisis meeting Saturday to discuss the situation at the two nuclear power plants.

Radiation measurements inside the Fukushima Daiichi nuclear 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 reported.

Prime Minister Naoto Kan toured the disaster area by helicopter.

Japan's nuclear authorities earlier informed the Vienna-based International Atomic Energy Agency (IAEA) that they have declared a nuclear emergency situation at Fukushima Daiichi, and had issued an alert for Fukushima Daini.

Citing Japanese official reports, the IAEA said the earthquake had cut the off-site power to the Fukushima Daiichi plant. Diesel generators meant to provide emergency power for the cooling system were disabled by flooding from the tsunami and had not yet been restored.

Fukushima Daiichi has three reactors. Their radioactive cores need continued cooling to prevent meltdown.

US Secretary of State Hillary Rodham Clinton said the US Air Force in Japan had transported coolant to the plant.