Japan raised the crisis level at its crippled nuclear plant Tuesday to a level severity on par with the 1986 Chernobyl disaster.
The government cited high overall radiation leaks that have contaminated the air, tap water, vegetables and seawater.
Japanese nuclear regulators said they raised the rating from 5 to 7 - the highest level on an international scale of nuclear accidents overseen by the International Atomic Energy Agency - after new assessments of radiation leaks from the Fukushima Dai-ichi plant since it was disabled by the March 11 tsunami.
The move signifies a "major accident" with widespread effects on the environment and health. But Japanese officials are downplaying the health concerns.
They say leaks from the plant so far amount to only a tenth of the radiation released in the Chernobyl disaster.
"This reconfirms that this is an extremely major disaster. We are very sorry to the public, people living near the nuclear complex and the international community for causing such a serious accident," Chief Cabinet Secretary Yukio Edano said.
But Edano told reporters there was no "direct health damage" from the crisis so far. "The accident itself is really serious, but we have set our priority so as not to cause health damage," he said.
Hironobu Unesaki, a nuclear physicist at Kyoto University Research Reactor Institute, said the revision was not a cause for worry, that it had to do with the overall release of radiation and was not directly linked to health dangers.
He said most of the radiation was released early in the crisis and that the reactors still have mostly intact containment vessels surrounding their nuclear cores.