BAGHDAD—Islamic State militants hiding among civilians have slowed an advance by U.S.-backed coalition forces in Syria, said the top American ground commander, a setback in the fight to reclaim the last vestiges of Islamic State’s territory.

Lt. Gen. Paul LaCamera, who commands the fight against Islamic State in Iraq and Syria, told reporters in Baghdad Sunday that it could take a day or several days to capture the territory. Other U.S. officials said it could take another one to two weeks or more.

“It’s an active battle,” Gen. LaCamera said. “There is a lot of fog and friction on the battlefield.”

President Trump on Friday had hinted that there would be an announcement about the end of fighting Islamic State’s physical caliphate within the next 24 hours, but multiple defense officials weren’t aware of any impending announcement and no timeline has been given to the Pentagon for troop withdrawal.

Gen. LaCamera and other officials declined to say how long it would take for the Syrian Democratic Forces to complete the job.