The operator of the Fukushima No. 1 nuclear power plant in Japan said Saturday extremely high radiation was detected at the No.1 reactor building of the plant.

The radiation readings were up to 4,000 millisieverts per hour at the building, the Tokyo Electric Power Co. (TEPCO) said.

Earlier, the company sent a robot into the No. 1 reactor building on Friday to inspect its inside.

TEPCO also said that steam was observed rising out of the floor of the reactor's building. The stream is probably coming from water accumulated in the basement of the reactor building.

The utility has been battling to keep the plant under control since it was crippled by the twin disasters in March.