BEIRUT: U.S.-backed Syrian militias said they seized a major dam on the Euphrates river from ISIS on Sunday, their latest gain as they push towards Raqqa city.

The Syrian Democratic Forces, an alliance of Kurdish and Arab fighters, said they captured the Baath Dam in the morning, renaming it Freedom Dam. The hydroelectric facility lies 22 kilometers upstream of Raqqa, where ISIS' operations in Syria are based.

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, which ISIS has used as a hub to plan attacks abroad.

The operation to storm the city will start in the next "few days", Mahmoud said on Saturday. The Raqqa assault will pile more pressure on the militant group's self-declared "caliphate" as it faces defeat in the Iraqi city of Mosul and retreats across much of Syria.

ISIS still controls swathes of Syria's eastern desert bordering Iraq and most of Deir al-Zor province, which would be its last major foothold in Syria after losing Raqqa.

The SDF has advanced in recent months to within several kilometers (miles) of the center of Raqqa, facing fierce resistance from ISIS.

Fighting since late last year has displaced tens of thousands of people, United Nations sources say, with many flooding camps in the area and others stranded in the desert.

The U.N. human rights office has raised concerns about increasing reports of civilian deaths as air strikes escalate. The Raqqa campaign has "resulted in massive civilian casualties, displacement, and serious infrastructure destruction" so far, it said in a May report.

ISIS militants have also reportedly forbidden civilians from leaving, it added.

The United States-led coalition says it is careful to avoid civilian casualties in its bombing runs in Syria and Iraq.