( CNN ) -- An explosion occured late Monday morning at the Fukushima Daiichi 's No . 3 nuclear reactor building , a government official told reporters . Chief Cabinet Secretary Yukio Edano said that the head of the nuclear plant reports that the container vessel , surrounding the reactor , remains intact . Japanese public broadcaster NHK showed images of white smoke rising above the facility , which is in northeastern Japan . Citing the nation 's nuclear and industrial agency , NHK said that a wall of one of the reactor 's buildings had collapsed . The 600 residents remaining within 20 kilometers of the plant , despite an earlier evacuation order , have been ordered to stay indoors , according to Edano . The secretary added that initial reports suggested that radiation levels had increased after the blast . But Edano said he did not believe there was a massive leak , given that water continues to be injected into the reactors and that the pressure inside the reactor is `` within a certain range . '' `` We are now collecting information on the concentration of radiation , '' Edano said . The facility is in the area of northeast Japan devasted by last week 's earthquake and tsunami .
The explosion occurred late Monday morning at the Fukushima # 3 reactor building . TEPCO said it was investigating what caused the blast.