Workers have been pulled out after black smoke is seen rising from the No. 3 reactor at troubled Fukushima No. 1 nuclear power plant in Japan, the operator of the utility said Wednesday.

The officials with the Tokyo Electric Power Co. said workers from the plant's No. 3 and No. 4 reactors have been evacuated for safety concerns.

The cause of the smoke is not yet known.

The Japanese authorities have been stepping up efforts to fix the nuclear power plant crippled by devastating March 11 earthquake and tsunami.