ANKARA, Turkey — Turkey said Tuesday that the U.S.-backed and Kurdish-led forces leading an assault on the Islamic State stronghold of Raqqa should not enter the city itself but merely help encircle it, a suggestion dismissed by the Kurds.

The dispute between the two U.S. allies threatens to complicate efforts to drive IS out of Raqqa, the de facto capital of the extremist group’s self-described caliphate.

The U.S.-backed Syria Democratic Forces, which include Kurdish, Arab and Turkmen fighters, have driven IS from large swaths of territory, but Turkey views the Syrian Kurds who dominate the group as an extension of the Kurdish insurgency raging in its southeast.

Turkey has sent its own forces across the border to back Syrian opposition fighters, and has suggested they lead the offensive to retake Raqqa. The Turkey-backed forces, now pushing toward the IS stronghold of al-Bab, have clashed with IS as well as the SDF.

The Britain-based Syrian Observatory for Human Rights said Turkey-backed opposition fighters captured six villages near al-Bab on Tuesday and are now about 4 miles (7 kilometers) from the town.

Turkish Foreign Minister Mevlut Cavusoglu told reporters that the U.S. Chairman of the Joint Chiefs of Staff, Gen. Joseph Dunford, told Turkish officials during a recent visit that Kurdish-led forces would only have a role in encircling Raqqa and would not enter the city.

“We hope that this will be the case and we expect that our partners keep their promises,” Cavusoglu said. He said “local” fighters aided by Turkish special forces should drive IS out of Raqqa, and suggested that residents of the mainly Sunni Arab city might not welcome Kurdish forces.