U.S.-backed Syrian militias said they fully captured the town of Tabqa and Syria's largest dam from Islamic State on Wednesday, a major objective as they prepare an assault on Raqqa city.

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

Nasser Haj Mansour, an adviser to the SDF, said the town and Tabqa dam were now "completely liberated" after the SDF drove all IS militants out of the area.

With air strikes and special forces from the U.S.-led coalition, the SDF have been advancing on Raqqa, Islamic State's base of operations in Syria, to isolate and ultimately seize the city.

Their campaign appeared to have stalled around Tabqa, where the SDF made only slow progress after besieging the town.

They pushed into the town earlier this month, capturing most of its districts and besieging Islamic State in part of the dam.

The battle for Tabqa began after U.S. forces helped SDF fighters conduct an airborne landing on the southern bank of the Euphrates in late March.