The hydroelectric dam lies some 22 km upstream of Raqqa, Islamic State's base of operations in Syria. The spokesman for the Kurdish YPG militia, the SDF's key component, said fighters were combing nearby villages for mines and shoring up their defensive lines. "The dam has been completely liberated," Nouri Mahmoud said. The advance means the SDF now hold all three major dams along the Euphrates, after gaining control of Syria's largest dam last month. With air strikes and special forces from the U.S.-led coalition, the SDF have been encircling Raqqa to take the city, which Islamic State has used as a hub to plan attacks abroad. The alliance advanced in recent months to within several kilometers (miles) of the center of Raqqa, facing fierce resistance from Islamic State. Fighting since late last year has displaced tens of thousands of people according to United Nations sources, with many flooding camps in the area. The operation to storm the city will start in the next "few days", Mahmoud had said on Saturday. The Raqqa assault will pile more pressure on the jihadist group's self-declared "caliphate" as it faces defeat in the Iraqi city of Mosul and retreats across much of Syria.