U.S.-backed militias said Sunday they had made a big advance in Tabqa, a strategically vital town controlling Syria's largest dam, in their campaign to drive Islamic State forces from its stronghold of Raqqa, 40 km (25 miles) downstream.

The Syrian Democratic Forces (SDF), a group of Kurdish and Arab militias, will wait to assault Raqqa until it seizes Tabqa, its military officials have previously said, but it has made only slow progress since besieging the town early in April.

In a statement it circulated on social messaging sites, the SDF said it had captured six more districts of Tabqa and distributed a map showing that Islamic State now controlled only the northern part of the town, next to the dam.