Tokyo, March 12 (DPA) The Japanese government Saturday said an explosion at a nuclear plant damaged by Friday's massive earthquake had not affected the reactors.Nor had the explosion at the Fukushima I plant, 240 kilometres north of Tokyo, led to a significant radioactive leak, Chief Cabinet Secretary Yukio Edano told a news conference.

The blast caused the roof of a building housing one of the plant's three reactors to collapse, injuring four people and raising initial fears of a disastrous meltdown.

The plant's operators, Tokyo Electric Power Co (TEPCO), said there was no damage to the steel containers in which the reactors are encased.

The government nevertheless extended the evacuation zone to residents living within 20 km of the plant.

Edano had earlier described the situation as "potentially very serious", but called on people to remain calm.

Technicians had been scrambling to reduce pressure in the reactors to prevent a meltdown, after the plant was damaged in Friday's massive quake, Japan's Nuclear and Industrial Safety Agency said.

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

TEPCO managed to depressurise the containers housing the reactors, the agency said, which involved relasing steam that would likely contain radioactive materials.

Technicians encountered difficulties opening a pressure release valve due to high radioactive levels, Kyodo news quoted the nuclear safety agency as saying.

Earlier Saturday, authorities had extended the evacuation zone to residents living within 10 km of the Fukushima I, where the cooling system experienced problems Friday.

But the evacuation had not been completed by early Saturday evening, Kyodo news said.

Meanwhile, residents within three kilometres of a second nuclear plant, Fukushima II located about 11 kilometres from Fukushima I, were also ordered to leave.

The government had held several crisis meetings Saturday to discuss the situation and Prime Minister Kan Naoto toured the disaster area by helicopter.

Radiation measurements inside the Fukushima I plant were 1,000 times higher than normal after the earthquake, Kyodo said.

Authorities were concerned that radioactivity may have escaped the plant due to high pressure inside an overheating reactor.

The magnitude-8.9 quake damaged power supplies and disrupted the reactors' cooling systems. An observation post near the plant's gate recorded radiation levels eight times higher than normal.

The cooling system for the three reactors at Fukushima II was also out of operation.

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

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

The radioactive cores of the reactors need continued cooling to prevent meltdown.

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