DAMASCUS, July 6 (Xinhua) -- At least 18 people were killed on Friday in a car bomb explosion in the eastern province of Deir al-Zour, a monitor group reported.

Half of the casualties were fighters with the U.S.-backed Syrian Democratic Forces (SDF), said the Syrian Observatory for Human Rights.

Meanwhile, a well-informed source told Xinhua that the car bomb went off at the water station in the town of Bsaira in the northern countryside of Deir al-Zour where the SDF units are positioned.

The SDF has been fighting the Islamic State (IS) on the eastern bank of Euphrates River in Deir al-Zour recently and the IS militants are believed to be behind the Friday explosion.

The U.S. has backed the SDF in their push to dislodge the IS from many areas in northern and eastern Syria, including Raqqa Province, the IS de facto capital in Syria, as well as in the southern countryside of Hasakah Province in northeastern Syria.

The SDF also fights the IS in the countryside of Deir al-Zour.

Despite being critical of the SDF alliance with the U.S., the Syrian government doesn't close the negotiation door with the group, which is made up of Arab, Kurdish and Assyrian fighters led by the Kurdish People's Protection Units (YPG).