A Syrian war monitor and a Kurdish official say members of the Islamic State group have killed 20 US-backed fighters in the country's east. The attack occurred late Friday in Deir el-Zour province where the US-backed Kurdish-led Syrian Democratic Forces launched a wide offensive this week to capture the last pocket held by IS in Syria. The Britain-based Syrian Observatory for Human Rights said the extremists took advantage of a sand storm to launch a counterattack, which killed 20 fighters and wounded others. Kurdish official Ebrahim Ebrahim said the 20 were killed in an ambush by IS fighters. The Observatory said Saturday that since the SDF launched its offensive on the IS-held pocket including the town of Hajin, 53 extremists have been killed as well as 37 US-backed fighters.