The agency says the cooling system at the number three reactor at the Fukushima nuclear power plant is offline and could possibly explode, following Saturday's blast at the plant's number one reactor.

The quake and tsunami cut the supply of off-site power to the plant and diesel generators intended to provide back-up electricity to the cooling system.

At the same time, Japanese engineers were focused on restoring the power supply to the stricken power plant in an attempt to reactivate its cooling system.