The Japanese nuclear safety agency has raised its level of alert at the damaged Fukushima plant from four to five on a scale of seven.
It puts the disaster on a par with the accident at Three Mile Island in the US in 1979.
Engineers hope to restore electricity this weekend to relaunch cooling operations and prevent a massive release of radiation.
If they fail, they could opt to cover the plant in concrete, the technique used at Chernobyl 25 years ago.
The government is continuing to insist there’s no serious risk to health outside the 30-kilometre exclusion zone.
The nuclear threat is overshadowing the widespread humanitarian crisis across the zone hit by the tsunami.
Emergency relief efforts are attempting to distribute water, food and other supplies to hundreds of thousands of people made homeless.
The official number of dead and missing continues to rise.