Earlier, Japanese Prime Minister Naoto Kan, making his first public statement on the crisis in a week, said the situation at the Fukushima nuclear complex, 240 km (150 miles) north of Tokyo, was "nowhere near" being resolved.

The comments reflected a spike of unease in Japan after several days of slow but steady progress in containing the nuclear accident, which was triggered by a devastating earthquake and tsunami on March 11.

Despite such a shocking toll, much attention since the disaster has been on the possibility of a catastrophic meltdown at Fukushima.

Two of the plant's six reactors are now seen as safe but the other four are volatile, occasionally emitting steam and smoke.

More than 700 engineers have been working in shifts to stabilise the plant and work has been advancing to restart water pumps to cool their fuel rods.

But fresh fears were raised when three workers trying to cool the most critical reactor were exposed to radiation levels 10,000 times higher than normally found in a reactor. They were hospitalized after walking in contaminated water though they are expected to be discharged soon.

The high level of contamination raised the possibility of a leak of radioactive material through a crack in the core's container which would mean a serious reversal following slow progress in getting the plant under control.