Japan's Nuclear Safety Agency spokesman Hideyuki Nishiyama said the workers were laying power cables to the plant, which was seriously damaged by a series of explosions in the days following the March 11 earthquake and tsunami.

External power supplies were restored to all six reactors at Fukushima on Tuesday. Engineers have been trying to restart the reactors' cooling systems to avert a major nuclear release but say the process could take weeks.

Firefighters have continued to spray water on the reactors to try to keep the nuclear cores cool.

The confirmed death toll from the 9.0-magnitude earthquake and subsequent tsunami now stands at 9,523, while another 16,094 people are listed as missing.

Police say the final death toll may exceed 18,000.