US airstrikes killed eight pro-Iran fighters in eastern Syria following a drone attack that killed one American contractor and injured five US service personnel, a war monitor said Friday.

"US strikes targeted a weapons depots inside Deir Ezzor city, killing six pro-Iran fighters, and two other fighters were killed by strikes targeting the desert of Mayadine and near al-Boukamal," said Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, a Britain-based monitor with a wide network of sources on the ground in the war-torn country.