A powerful explosion on Monday knocked down part of a five-story building that houses military families in the largest city in southeast Turkey, authorities said. There were no immediate reports of casualties, but ambulances rushed to the scene and rescuers clawed through the rubble. Authorities were investigating the cause of the blast in the city of Diyarbakir, in a region where Turkish troops have battled separatist Kurdish rebels for decades. Television reports said the blast occurred in the basement of a building where a heating boiler is located. The building is inside the perimeter of a heavily guarded military compound.