Islamic State militants killed at least 40 Syrian fighters backed by the United States, captured several alive and regained areas they lost earlier this month in eastern Syria near the Iraqi border in some of the most intense fighting in weeks, a war monitor and an agency linked to the extremist group reported on Saturday.

Members of the Syrian Democratic Forces, or SDF, have been on the offensive since early September under the cover of airstrikes by the American-led coalition to capture the last pocket held by the Islamic State in Syria.

Friday’s fighting lasted until the early hours of Saturday began when Islamic State militants, taking advantage of a sandstorm, launched a counteroffensive against the Syrian Democratic Forces in the eastern province of Deir el-Zour that borders Iraq, activists said.

Colonel Sean Ryan, spokesman for the American-led coalition, said they have no confirmation of exact figures “as both sides are taking casualties.” He called it a “difficult fight.”