Japan's nuclear safety agency has raised the severity rating of the crisis at its nuclear plant to
the highest level, on par with the 1986 Chernobyl disaster.
An official with the Nuclear Safety Commission of Japan, speaking on national television, said on Tuesday the rating was raised from five to seven on an international scale of atomic crises.
The official, who was not named, said the amount of radiation leaking from the Fukushima Daiichi nuclear plant was about 10 per cent of that in the Chernobyl accident.
"In terms of volume of radioactive materials released, our estimate shows it is about 10 percent of what was released by Chernobyl," the nuclear safety agency spokesman told reporters.
The crisis at the crippled nuclear plant was sparked by last month's earthquake and tsunami, which killed more than 13,000 people, with about 14,500 people still missing.
The upgrading to a "major accident" puts Fukushima on a par with the 1986 Chernobyl disaster, the world's worst ever peacetime nuclear event.
The meltdown at Chernobyl in the then-Soviet Union spewed a large volume of toxic radiation, poisoning large areas of land and affecting thousands of lives.
Workers at Fukushima Daiichi are battling to control overheating reactors whose cooling systems were knocked out by last month's enormous waves.
The re-assessment from level five on the scale came as Japan was rocked by yet another powerful aftershock from the 9.0 magnitude quake that unleashed a devastating tsunami on March 11.
The 6.2 magnitude tremor hit 77 kilometres east of Tokyo on Tuesday and swayed buildings in the capital, temporarily shutting down subway services and halting bullet trains. US geologists originally put the magnitude at 6.4.
In a fresh setback, a fire broke out early Tuesday morning at a battery unit outside a building at the No.4 reactor unit at the Fukushima Daiichi plant, but was quickly extinguished and did not spread to other areas, plant operator Tokyo Electric Power said.
The operator said it was not related to Tuesday's quake.
