Officials scrambled to contain the nuclear crisis with a variety of patchwork fixes. The top US nuclear regulator warned that one reactor's cooling pool for spent fuel rods may have run dry and another was leaking.

"We believe that around the reactor site there are high levels of radiation," Gregory Jaczko, head of the Nuclear Regulatory Commission, told a House Energy and Commerce subcommittee hearing.

Health experts said panic over radiation leaks from the Daiichi plant was also diverting attention from other threats to survivors of Friday's 9.0 magnitude quake and tsunami, such as the cold or access to fresh water.

The head of the world's nuclear watchdog, meanwhile, said it was not accurate to say things were "out of control" in Japan, but the situation was "very serious", with core damage to three units at the plant, around 240 kms (150 miles) north of Tokyo.

The latest photographs from the plant showed severe damage to some of the buildings after several blasts.

G7 Finance ministers will hold a conference call later on Thursday to discuss steps to help Japan cope with the financial and economic impact of the disaster, a source said.

Japan's government said radiation levels outside the plant's gates were stable but, in a sign of being overwhelmed, appealed to private companies to help deliver supplies to tens of thousands of people evacuated from around the complex.