“We've confirmed that the reactor container was not damaged. The explosion didn't occur inside the reactor container. As such there was no large amount of radiation leakage outside,” he said.

“At this point, there has been no major change to the level of radiation leakage outside (from before and after the explosion), so we'd like everyone to respond calmly,” Edano said.

“We've decided to fill the reactor container with sea water. Trade minister Kaieda has instructed us to do so. By doing this, we will use boric acid to prevent criticality.”

Edano said it would take about five to 10 hours to fill the reactor core with sea water and around 10 days to complete the process.

Edano said due to the falling level of cooling water, hydrogen was generated and that leaked to the space between the building and the container and the explosion happened when the hydrogen mixed with oxygen there

Radiation leaked from an unstable Japanese nuclear reactor north of Tokyo on Saturday, the government said, after an explosion blew the roof off the facility in the wake of a massive earthquake.

The developments raised fears of a disastrous meltdown at the plant, which was damaged by Friday's 8.9-magnitude earthquake, the strongest ever recorded in Japan.

“We are looking into the cause and the situation and we'll make that public when we have further information,” Chief Cabinet Secretary Yukio Edano said.

Edano said an evacuation radius of 10 kilometres from the stricken 40-year-old Daiichi 1 reactor plant in Fukushima prefecture was adequate. TV footage showed vapour rising from the plant, 240 kilometres north of Tokyo.

The quake sent a 10-metre tsunami ripping through towns and cities across the northeast coast. Japanese media estimate that at least 1,300 people were killed.

The blast came as plant operator Tokyo Electric Power Co (TEPCO) worked desperately to reduce pressures in the core of the reactor.

“An unchecked rise in temperature could cause the core to essentially turn into a molten mass that could burn through the reactor vessel,” risk information service Stratfor said in a report before the explosion. “This may lead to a release of an unchecked amount of radiation into the containment building that surrounds the reactor.”

NHK television and Jiji said the outer structure of the building that houses the reactor appeared to have blown off, which could suggest the containment building had already been breached.

Earlier the operator released what it said was a tiny amount of radioactive steam to reduce the pressure and the danger was minimal because tens of thousands of people had already been evacuated from the vicinity.

Reuters journalists were in Fukushima prefecture, about 70 kilometres from the plant. Other media have reported police roadblocks in the area to prevent people getting closer.

Italy's National Institute of Geophysics and Volcanology said the earth's axis shifted 25 centimetres as a result of the quake and the US Geological Survey said the main island of Japan had shifted 2.4 metres.

Friday's tremor was so huge that thousands fled their homes from coastlines around the Pacific Rim, as far away as North and South America, fearful of a tsunami.

Most appeared to have been spared anything more serious than some high waves, unlike Japan's northeast coastline which was hammered by the huge tsunami that turned houses and ships into floating debris as it surged into cities and villages, sweeping aside everything in its path.

“I thought I was going to die,” said Wataru Fujimura, a 38-year-old sales representative in Koriyama, Fukushima, north of Tokyo and close to the area worst hit by the quake.

“Our furniture and shelves had all fallen over and there were cracks in the apartment building, so we spent the whole night in the car...now we're back home trying to clean.”

The unfolding natural disaster, which has been followed by dozens of aftershocks, prompted offers of search and rescue help from 50 countries.

The central bank said it would cut short a two-day policy review scheduled for next week to one day on Monday and promised to do its utmost to ensure financial market stability.

The disaster struck as the world's third-largest economy had been showing signs of reviving from an economic contraction in the final quarter of last year. It raised the prospect of major disruptions for many key businesses and a massive repair bill running into tens of billions of dollars.

In one of the worst-hit residential areas, people buried under rubble could be heard calling out for rescue, Kyodo news agency reported. TV footage showed staff at one hospital waving banners with the words “FOOD” and “HELP” from a rooftop.

The airport in coastal city Sendai, home to one million people, was on fire, Japanese media said.

TV footage from Friday showed a black torrent of water carrying cars and wrecked homes at high speed across farmland near Sendai, 300 kilometres northeast of Tokyo. Ships had been flung onto a harbour wharf, where they lay helplessly.

The earthquake was the fifth most powerful to hit the world in the past century. It surpassed the Great Kant quake of September 1, 1923, which had a magnitude of 7.9 and killed more than 140,000 people in the Tokyo area.

The 1995 Kobe quake caused $100 billion in damage and was the most expensive natural disaster in history.