THERE has been an explosion at the earthquake-hit nuclear plant at Fukushima, Japan, with several workers reported injured inside.

The Japanese government confirmed shortly after 9.30am that there had been a radiation leakage.

An initial 10km exclusion zone was extended to 20km around the unstable reactor, 150 miles to the north of Tokyo.

The roof is reported to have been blown off the plant, one of the biggest in the world, sparking fears of a nuclear meltdown at the 40-year-old Daichi 1 reactor.

However, it hoped that the explosion will have been contained.

Footage on Japanese TV showed that the walls of one building had crumbled, leaving only a skeletal metal frame block standing.

Puffs of smoke were spewing out of the plant.

Pressure has been building up in the reactor - it's now twice the normal level - and Japan's Nuclear and Industrial Safety Agency told reporters today that it was venting ''radioactive vapours'' to relieve that pressure.

Officials said they were measuring radiation levels in the area.

The reactor in trouble has already leaked radiation: Operators at the plant's Unit 1 detected eight times the normal radiation levels outside the facility and 1,000 times normal inside Unit 1's control room.

''We are now trying to analyse what is behind the explosion,'' said government spokesman Yukio Edano, stressing that people should quickly evacuate a six-mile radius."

The trouble began at the plant's Unit 1 after Friday's massive 8.9-magnitude earthquake and the tsunami it spawned knocked out power there.

The disaster has killed hundreds of people and devastated the country's north-eastern coast, where rescuers began slowly arriving today.

The toll of destruction was still not known more than 24 hours after the quake since washed-out roads and shut airports have hindered access to the area.

An untold number of bodies were believed to be buried in the rubble and debris.

The official death toll stood at 413, while 784 people were missing and 1,128 injured. In addition, police said between 200 and 300 bodies were found along the coast in Sendai, the biggest city in the area near the quake's epicentre.

Local media reports said at least 1,300 people may have been killed.

Wind in the region is weak and headed north-east, out to sea, according to the Meteorological Agency.

Ryohei Shiomi, an official with Japan's nuclear safety commission, said that even if there was a meltdown, it wouldn't affect people outside a six-mile radius - an assertion that might need revising if the situation deteriorates.