BEIRUT—Islamic State killed dozens of U.S.-backed fighters in Syria over the weekend, a war monitor said, highlighting the complications for an American troop withdrawal from the country as the extremist group fiercely defends its last positions.
At least 91 fighters belonging to the Kurdish-led Syrian Democratic Forces died in the fight that started on Friday in eastern Deir Ezzour province, the U.K.-based Syrian Observatory for Human Rights said on Monday. About 61 militants and 51 civilians, mostly family members of Islamic...