Hundreds of people were taken by ISIS militants from a displacement camp in east Syria during a counterattack against advancing US-backed forces, the Syrian Observatory for Human Rights said Saturday. The extremists raided the camp on Friday, taking "more than 100 families" including relatives of ISIS defectors and of militants killed in fighting, according to the monitor. A number of fighters of the Kurdish-led Syrian Democratic Forces (SDF) died trying to defend the camp in a battle that lasted several hours, the Britain-based group said. The US-backed SDF began on September 10 a final push on the small stretch of the Euphrates Valley around the town of Hajin, near the Iraq border, where they estimate some 3,000 militants are holed up. But they have sustained heavy casualties in the operation being conducted with US-led air support. Since Wednesday, 37 SDF fighters have been killed in militant counterattacks, the Observatory said. ISIS has lost 58 fighters, most of them in retaliatory coalition air strikes, it added. "ISIS is pressing its attacks in the Hajin area as the SDF battles to hold them off with the support of the international coalition," Observatory chief Rami Abdel Rahman told AFP. But a prolonged sandstorm and bad visibility have made it difficult for the coalition to carry out air strikes. ISIS published statements on its social media accounts describing a number of attacks by its members against SDF forces in the Hajin area.  The battle for Hajin has claimed the lives of 176 SDF fighters and 325 terrorists since its launch last month, according to Observatory figures. The SDF, backed by the coalition, has ousted ISIS from swathes of northern and eastern Syria, including Raqqa, the de facto capital of ISIS. But its not clear how many militants are still fighting with the terrorist organization. A United Nations report released in August said ISIS has up to 30,000 members distributed roughly equally between Syria and Iraq, and said its global network increasingly poses a threat.