A car bombing in eastern Syria on Friday killed at least 18 people including 11 members of the U.S.-backed Syrian Democratic Forces (SDF), a war monitor said.

"A car bomb went off in front of the Syrian Democratic Forces' base in Al-Bsayra town in the eastern Deir el-Zour countryside," said Rami Abdel Rahman, head of the Syrian Observatory for Human Rights.

It killed "a commander and ten other personnel, as well as seven civilians including three children," he said.

The U.S. has partnered with the SDF, predominantly led by the PKK-affiliated People's Protection Units (YPG), in their fight against Daesh terror group, despite warnings from Turkey against the repercussions of using one terrorist group to defeat another.