The US-backed Syrian Democratic Forces said Tuesday it has begun the final stage of its campaign to clear ISIS militants from northeast Syria as 21 pro-regime gunmen have been killed in clashes with the terrorist group in the countrys south. The SDF said it launched operations on Monday to expel ISIS militants from the town of Hajin and surrounding villages on the northeast banks of the Euphrates River. The SDF is supported by the US-led international coalition, which said in a statement it was supporting the SDF ground operation with air and artillery strikes. The Britain-based Syrian Observatory for Human Rights monitoring group said coalition jets bombed locations across the ISIS pocket in conjunction with the SDF ground offensive. At least 23 ISIS extremists were killed in the first 24 hours of battle, according to the Observatory. Despite losing 90 percent of its territory between Iraq and Syria since its 2014 peak, the militant group remains a disruptive force in both countries. In July, the group ambushed Sweida province in south Syria, sparking a day of battles that killed some 300 people. The militants continue to hold captive approximately 30 members of the minority Druze sect that they kidnapped during the July battle. The Observatory said 21 pro-regime gunmen died in clashes with the militants Monday night in the Safaa region in the Syrian desert. Regime forces backed by local and regional militias have been battling the militants for weeks in the countryside north of Sweida since the militants ambushed the city and its environs in July. The militants are believed to be holding the captives from the Druze community in the Safaa area.