BEIRUT (AP) — Syrian opposition activists said Monday that U.S.-backed Syrian fighters have killed nine civilians, including six children, who were trying to flee the last area controlled by the Islamic State group in eastern Syria.

It was not immediately clear if the Kurdish-led Syrian Democratic Forces was behind the shooting that killed the three women and six children in the eastern province of Deir el-Zour. An SDF spokesman did not immediately respond to request for comment.

The SDF has captured, with U.S. military support, wide areas in eastern Syria from the Islamic State group.

More than 30,000 civilians, IS fighters and their families have fled zones controlled by the extremists and many have been moved to a tent settlement in northeastern Syria. Months of fighting have left hundreds of civilians and fighters dead on both sides.

The Britain-based Syrian Observatory for Human Rights said the incident occurred Saturday night, adding that the gunfire came from SDF positions. It said the civilians had paid money to smugglers to take them out of the area controlled by the extremists.

The DeirEzzor 24, an activist collective, said the incident occurred near Tanak oil field, which is close to the front line between IS and the SDF, blaming the Kurdish-led force for the shooting.

The SDF said in a statement Monday that it captured three foreign IS members — a German, a Saudi Arabian and an Egyptian — on Feb. 1.

The Kurdish-led force is holding more than 700 non-Syrian IS fighters from more than 40 countries. Few have been returned to their home nations.