Workers at Japan's tsunami-damaged nuclear power plant temporarily halted work on Wednesday at the complex's reactor No 2 because of high radiation levels, the nation's nuclear safety watchdog said.

The Nuclear and Industrial Safety Agency said the workers had withdrawn after radiation monitoring showed levels of 500 millisieverts per hour, which is into the danger zone.

There are hundreds of workers at the plant trying to avert nuclear disaster and, at times, they have had to withdraw from parts of the complex as radiation levels spike.