U.S.-backed Syrian militias said they fully seized the town of Tabqa and Syria’s largest dam from Islamic State on Wednesday, a major objective as they prepare to launch an assault on Raqqa, the jihadists’ biggest urban stronghold.

The Syrian Democratic Forces (SDF), an alliance of Kurdish and Arab fighters, have been battling the militant group for weeks in Tabqa, some 40 km (25 miles) west of Raqqa, along the Euphrates River.

With air strikes and special forces from the U.S.-led coalition, the SDF are advancing on Raqqa to ultimately take the city, which is also the Islamic State’s base of operations in Syria.

They captured Tabqa “thanks to the sacrifices of the SDF’s heroes and with the full, unlimited support of the U.S.-led international coalition”, said SDF spokesman Talal Silo.