Japan scrambled on Saturday to avert a disastrous meltdown at a nuclear plant damaged when a massive earthquake and tsunami struck the northeast coast.
Jiji news agency said there had been an explosion at the tricken 40-year-old Daichi 1 reactor and TV footage showed vapour rising from the plant, which lies 240 km (150 miles) north of Tokyo.
The country's nuclear safety agency could not confirm the reported incident, which came as plant operator Tokyo Electric Power Co (Tepco) worked desperately to reduce pressures in the core of the reactor that - if not contained - could lead to a release of radiation into the atmosphere.
"An unchecked rise in temperature could cause the core to essentially turn into a molten mass that could burn through the reactor vessel," political risk information service Stratfor said in a report. "This may lead to a release of an unchecked amount of radiation into the containment building that surrounds the reactor."
NHK television 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.
Media reports estimate at least 1,300 people may have been killed by the 8.9 quake, the biggest since records began in Japan 140 years ago, and the 10-metre tsunami that swept ferociously inland after it struck.
Italy's National Institute of Geophysics and Volcanology said the earth's axis had shifted 25 cm as a result of the quake and the US Geological Survey said the main island of Japan had actually shifted 2.4 metres.
Japanese officials and experts have been at pains to say that while there would be radiation leaks, they would be very small and have dismissed suggestions of a repeat of a Chernobyl-type disaster.
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 Watauga Fuji, 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.
The earthquake was the fifth most powerful to hit the world n 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.