Footage of the blast at the Fukushima Daiichi No 1 nuclear plant in Japan which is under a state of emergency after the earthquake and tsunami Reuters

Japan is battling to stave off a nuclear disaster after an explosion at a north-eastern nuclear plant in the wake of the enormous earthquake and tsunami.

Authorities are evacuating tens of thousands of residents living within a 12 mile (20km) radius of the Fukushima Daiichi plant and those within 6 miles of a second installation in Futuba, 150 miles north of Tokyo.

The explosion followed warnings of a possible meltdown after problems with the cooling system and confirmation of a radiation leak at Fukushima No 1 plant. But nuclear safety officials said it was unlikely the reactor had suffered serious damage, according to the Kyodo news agency.

It is feared that 1,300 people died in Friday's double disaster, most being killed as the wall of mud and water engulfed buildings, roads and vehicles, Japanese media reported. But the priority now is to tackle the crisis at the power plant.

Kyodo cited an official who said that the rate of hourly radiation leaking from Fukushima was equal to the amount usually permitted in a year.

Authorities had previously heralded a successful release of radioactive gases to reduce pressure inside the reactor, which might account for the high levels.

"We are now trying to analyse what is behind the explosion," said the chief cabinet secretary, Yukio Edano. "We ask everyone to take action to secure safety."

Television footage showed the walls of one building had crumbled, leaving only its metal frame, but it was not clear whether it housed the reactor.

The Tokyo Power Electric Company, which runs the Fukushima Daiichi plant, said four workers were injured in the explosion.

Hours after the blast, officials widened a 6-mile evacuation zone around the plant and around Fukushima No 2 plant.

The Tokyo fire department has dispatched an elite Hyper rescue team to the nuclear plant.

An uncontrolled temperature rise at the plant could lead to a meltdown of the uranium reactor core. This could burn through the walls of the vessel and release radiation into a containment building that surrounds the reactor. Some fuel is already thought to have melted in the reactor.

Japanese media said officials had detected iodine and caesium, elements released when overheating causes core damage.

The UN nuclear watchdog, the International Atomic Energy Agency, said it was urgently seeking information.

An explosion of the pressure vessel at the Chernobyl reactor in 1986 led to a vast release of radiation.

But experts and authorities urged people to remain calm, suggesting the chances of a major disaster were slight.

The crisis began when the 8.9 magnitude earthquake cut out power, turning off the water supply needed to cool the system. The tsunami is thought to have cut off the backup diesel generator an hour later, leading to pressure rising rapidly within the reactor.

Earlier in the day a Japanese nuclear safety panel said radiation levels were 1,000 times higher than normal in a control room and eight times higher than normal just outside the plant.

Speaking before the blast, Naoto Sekimura, a professor at the University of Tokyo, told the Associated Press a major radioactive disaster was unlikely.

"No Chernobyl is possible at a light water reactor. Loss of coolant means a temperature rise, but it also will stop the reaction," he said.

"Even in the worst-case scenario, that would mean some radioactive leakage and equipment damage, but not an explosion. If venting is done carefully, there will be little leakage. Certainly not beyond the 3km radius."

A partial meltdown in one of the light water reactors at Three Mile Island in 1979 resulted in the release of radioactive gases in the most serious incident in the history of the US nuclear power industry. The reactor was eventually brought under control despite a series of errors.

The blast has compounded the fears of survivors in the worst hit region, north-eastern Tohoku, where aftershocks continue to rock the ground.

@DavidHalton in Sendai city tweeted: "Constant sirens and aircraft that I hope are military that on top of worrying about nuclear fallout and tremors."

Residents woke up after a freezing night on rooftops and in emergency shelters to a sea of mud, water and debris. Earthquakes continued to rock the north-east coast overnight, although some said the worst tremors appeared to be subsiding.

Kyodo said rail operators had yet to find four trains after losing contact with them as they operated on coastal lines on Friday.

East Japan Railway Company said it did not know how many people were on board the trains.

Japan downgraded tsunami warnings in most areas but Tohoku remained on high alert for waves of up to 10 metres high.

The tsunami has reached countries across the Pacific region but there were no reports of major damage outside Japan.

The country has mobilised 50,000 rescuers, and footage showed some winching people to safety from rooftops.

Witnesses said the tsunami had swept inland by up to six miles in Sendai, which has around 1 million inhabitants and is 80 miles from the epicentre.

"The flood came in from behind the store and swept around both sides. Cars were flowing right by," said Wakio Fushima, who owns a grocery shop.

"The tsunami was unbelievably fast. Smaller cars were being swept around me and all I could do was sit in my truck," said driver Koichi Takairin, who was trapped in his four-tonne vehicle by the torrent.