The Nuclear and Industrial Safety Agency said Saturday that radioactive cesium has been detected at Fukushima No. 1 nuclear power plant.

At a press conference, agency officials said that the detection indicates a reactor meltdown is occurring.

Workers were struggling to cool the facility and restart its quake-damaged diesel generators to avoid a total meltdown, officials said.

Friday's massive 8.8 magnitude earthquake and tsunami that struck northeastern Japan cut the supply of off-site power to the plant and diesel generators intended to provide back-up electricity to the cooling system.

Experts warned there was the possibility of a core meltdown, which would have devastating effects for millions of people and the environment.

He said once a core meltdown began there would be little anyone could do to prevent radioactive material escaping.

Pressure was stable inside the reactors of Fukushima No. 2 plant but rising in the containment vessels, a spokesman for the operator said.

Earlier in the day authorities expanded evacuation zone from a radius of 3 km to 10 km for residents in the vicinity of the No. 1 plant, following the amount of radiation reaching 1,000 times the regular level in the operating room.

Japan's Ground Self Defense Forces arrived Saturday to ensure the safe evacuation of local people living near the two stricken nuclear plants.

Chief Cabinet Secretary Yukio Edano said earlier on Saturday that releasing radioactive steam to reduce the pressure would not cause any immediate threat to people's health and called for people to stay calm.