An explosion has blown the roof off an unstable reactor north of Tokyo, raising fears of a disastrous meltdown at a nuclear plant damaged in the massive earthquake that hit Japan on Friday.

The 8.9-magnitude quake, the strongest ever recorded in Japan, sent a 10-metre tsunami ripping through towns and cities across the north-east coast.

Japanese media estimate that at least 1,300 people have been killed.

An explosion at the stricken 40-year-old Fukushima No 1 nuclear power plant was heard about 3:30pm (local time) following a series of large tremors.

TV footage showed thick white smoke rising from the plant, which lies 240 kilometres north of Tokyo.

Public broadcaster NHK and Jiji report the outer structure of the building that houses the reactor has blown off.

Four workers - two from operator Tokyo Electric Power Co and two others from another firm - were injured in the explosion.

Tokyo Electric says the injuries they have suffered are not life-threatening and that all four are conscious.

A government spokesman says a Tokyo Fire Department rescue team has been sent to the site.

Chief cabinet secretary Yukio Edano has confirmed a radiation leak and says the government is stockpiling iodine as part of a contingency plan.

He urged people to stay calm and says the government is carefully monitoring the radiation level.

The UN nuclear watchdog says it is aware of the explosion at the No 1 plant and is urgently seeking information from the country's authorities.

According to the Fukushima prefectural government, the hourly radiation from the plant reached 1,015 microsievert - an amount equivalent to that allowable for ordinary people in one year.

Nearby residents are being warned to stay indoors, turn off air-conditioners and not to drink tap water.

If people must go outside, they should avoid exposing their skin and cover their face with masks and wet towels.

More than 45,000 residents within a 10-kilometre radius of the plant had been evacuated, but the evacuation area has now been expanded to 20-km radius from the plant.

Thousands more have been evacuated from near a second plant, Fukushima No 2, which also suffered damage to its cooling system following Friday's quake.

Graham, an English teacher living near the Fukushima nuclear plant, says people are stocking up on supplies as they prepare to leave the area.

"People are stocking up on batteries, water food, anything warm. There's a little bit of, I wouldn't say panic, but the supermarkets were just crazy," he said.

"Hopefully there's no sort of worries with radiation and everything at the moment, but obviously it's a concern."

Steve Kerekes, a nuclear expert from the US Nuclear Energy Institute, warns if technicians cannot get the reactor's cooling systems working there is a possibility of radiation leaks similar to America's worst nuclear accident.

"If indeed that were to happen at some point you're going to have sufficient evaporation in the core that, if you will, your fuel is going to heat up," he said.

"Then you'll have a meltdown not unlike what happened in the United States Three Mile Island plant in Pennsylvania back in 1979."

Meanwhile Japan's Meteorological Agency says Fukushima prefecture is at high risk of another major tsunami.

ABC/wires