The Nuclear and Industrial Safety Agency said Saturday that radioactive cesium has been detected at Fukushima No. 1 nuclear power plant, adding that the detection indicates a reactor meltdown is occurring.

The detection of the leak came after authorities successfully completed the expansion of a 3-km evacuation zone to 10-km for residents in the vicinity of the No. 1 plant earlier in the day, following the amount of radiation reaching 1,000 times the regular level in the operating room.

Earlier in the day, the government issued an evacuation order for residents in the vicinity of Fukushima No. 2 nuclear plant, following a magnitude 8.8 earthquake that hit northern Japan Friday causing the plant to malfunction.

The nuclear safety agency said earlier Saturday that residents in the vicinity were not in immediate danger from radioactive leaks, but the plant's operator is set release pressure in containers housing nuclear reactors that could result in the leak of radioactive substances.

The order to release the pressure was issued by the government in a never-before-seen move to try and stabilize the faltering plant.

The order to expand the evacuation zone came from Prime Minister Naoto Kan, who visited the facility Saturday.

The Nuclear and Industrial Safety Agency has been monitoring the radiation levels closely due to the possibility of radioactive steam spreading around the plant.

At the entrance to the plant radiation was detected at eight times the normal level, the agency said.