The international coalition struck back against an attack on a Syrian Democratic Forces (SDF) stronghold in Deir Ezzor province, killing at least 100 pro-regime fighters, AFP reported Thursday (February 8th).

The initial attack, carried out by pro-regime forces, targeted key oil and gas installations in parts of Deir Ezzor province controlled by the Arab-Kurd alliance.

According to the US Central Command, coalition advisers were present in the area that was attacked by pro-government forces in Deir Ezzor province late on Wednesday.

"The coalition conducted strikes against attacking forces to repel the act of aggression" against its own personnel and the SDF hosting them, it said.

"We estimate more than 100 Syrian pro-regime forces were killed while engaging SDF and coalition forces," a US military official said on condition of anonymity.

The SDF and the coalition targeted the attacking forces with airstrikes and shelling after "20 to 30 artillery and tank rounds landed within 500 metres of the SDF headquarters location", the official said.

According to the Syrian Observatory for Human Rights, which only confirmed 45 dead among pro-regime forces, the initial attack took place near Khasham.

Observatory head Rami Abdel Rahman said the aim of the attack appeared to be the capture of a key oil field and a major gas plant in an SDF-held area.

The Omar oil field, one of the biggest in Syria, had a pre-war output of 30,000 barrels per day, while the Conoco gas field had a pre-war capacity of 13 million cubic metres a day.

Syrian state media confirmed that dozens were killed in the clash but appeared to deny the forces were army soldiers, describing the victims as "popular forces".

According to the Observatory, the forces that launched the attack on SDF positions were local tribal fighters loyal to Assad and Afghan Shia militia fighting alongside the regime.

The US said the attack occurred eight kilometres east of the "Euphrates River de-confliction line", referring to a boundary agreed by Russia and the US, with the former's area of operations west of the river and the latter's to its east.