BEIRUT — U.S.-backed Syrian forces liberated the city of Raqqa from Islamic State militants on Tuesday, a senior commander for the force said, adding that clearing operations were underway to remove land mines left behind and search for the extremist group's sleeper cells. Brig. Gen. Talal Sillo told The Associated Press that there are no longer clashes in the city, which had served as the extremist group's headquarters and self-proclaimed capital of their so-called "caliphate" for more than three years. A formal declaration will be made from the city soon, after the clearing operations end.