Japanese authorities have increased the severity rating of their nuclear crisis to the highest level, seven.

They said the decision reflects the total release of radiation at the Fukushima Daiichi power plant, rather than a sudden deterioration.

There have been no fatalities resulting from the leaks at the plant, and risks to human health are reported to be low.

Level seven previously only applied to the 1986 Chernobyl disaster, where 10 times as much radiation was emitted. In Chernobyl it was the reactor core itself that exploded, releasing a huge amount of radioactive material in a very short space of time. Whereas Fukushima experienced a less critical hydrogen explosion.

Meanwhile a 6.0-magnitude earthquake on Tuesday led to a staff evacuation of the plant. The earthquake also rumbled buildings in Tokyo.

There were no immediate reports of fresh damage, though Japan’s Narita international airport temporarily closed its runways, and metro and train services were interrupted.

On Monday, a 7.1-magnitude quake hit north-east Japan, leaving three people dead. It also triggered a brief tsunami warning, and forced workers to evacuate the Fukushima Daiichi plant.

The aftershocks come a month after a huge quake and tsunami hit north-east Japan, leaving 13,228 people dead and 14,529 missing. More than 150,000 people have been made homeless.

“We have upgraded the severity level to seven as the impact of radiation leaks has been widespread from the air, vegetables, tap water and the ocean,” said Minoru Oogoda of Japan’s Nuclear and Industrial Safety Agency (Nisa), the government’s nuclear watchdog.

The decision to raise the threat level was made after radiation of a total up to 630,000 terabequerels had been estimated.

That would classify the crisis at level seven on the International Nuclear and Radiological Event Scale (Ines).

The severity level of Japan’s nuclear crisis had previously been set at five, the same as that of the accident at Three Mile Island in the US in 1979.

Japan has also said it is extending the evacuation zone around the crippled nuclear plant because of radiation concerns.