A war monitor says the US airstrikes on positions of Iran-backed militias in eastern Syrian city of Deir ez-Zur (Deyrizor) have killed eights militiamen, after a deadly drone strike targeted American forces in the area.

The Britain-based Syrian Observatory for Human Rights said the attacks were carried out in the early hours of Friday local time by US fighter jets and drones, targeting positions of the Iranian Revolutionary Guards (IRGC) in Al-Bokamal desert, as well as other positions on the southern outskirts of Al-Mayadeen city and an ammunition warehouse opposite the “Officers Residences” in Harabesh neighborhood in Deir ez-Zur city.

The airstrikes left at least eight Iranian-backed militiamen dead: six in the warehouse in Harabesh and two in the position on the outskirts of Al-Mayadeen. According to the observatory, the number of fatalities is expected to rise as the strikes left several other militiamen injured, some seriously. The strikes also caused heavy material damage.

Also Friday morning -- at around 11 a.m. local time (0800 GMT) -- Iran-backed forces stationed in a base near Ain Ali shrine in Al-Mayadeen countryside fired several missiles towards Al-Omar oilfield which hosts the US-led International Coalition’s largest base in Syria. 

The Pentagon said earlier in the day that the US military carried out the attack against militia groups who it blamed for a drone attack on Thursday that killed an American contractor, injured another, and wounded five US troops. 

The US intelligence community assessed that the one-way attack drone was Iranian in origin, the military said, a conclusion that could further aggravate already strained tensions between Washington and Tehran.