WASHINGTON: The US-led anti-jihadist coalition carried out strikes in "self-defense" against forces loyal to Syrian President Bashar al-Assad, leaving an estimated more than 100 pro-regime personnel dead, the US military said Wednesday.
The strikes were to counter an "unprovoked attack against well-established Syrian Democratic Forces headquarters," the US Central Command said, referring to the coalition's Syrian allies, who have played a major role in the war against the Islamic State jihadist group.
"We estimate more than 100 Syrian pro-regime forces were killed while engaging SDF and coalition forces," while one SDF member was wounded, a US military official said.
According to CENTCOM, "coalition service members in an advise, assist, and accompany capacity were co-located with SDF partners during the attack."
"The coalition conducted strikes against attacking forces to repel the act of aggression," it said.
The US military official said the attack -- which began on Wednesday -- involved some 500 personnel from "pro-regime" forces armed with tanks, artillery and mortars.
The SDF and the coalition targeted the attacking forces with air and artillery strikes after "20 to 30 artillery and tank rounds landed within 500 meters (yards) of the SDF headquarters location," the official said.
The official did not specifically identify the attacking forces, which could have been Syrian or from one of a number of allied militia units, including from Iraq and Lebanon, that are backing Assad in the country's seven-year civil war.
Russia is carrying out strikes in support of Assad, who has received substantial support from Iran as well.
CENTCOM said the attack occurred eight kilometers (five miles) east of the "Euphrates River de-confliction line," referring to a boundary agreed to by Russia and the US, with the former's area of operations to the west of the river and the latter's to its east.
"Coalition officials were in regular communication with Russian counterparts before, during and after the thwarted" attack, the US military official said, adding that "Russian officials assured coalition officials they would not engage coalition forces in the vicinity."
The SDF -- an alliance of Kurdish and Arab fighters -- is working with coalition support to uproot the last IS jihadists from the east bank of the Euphrates, after capturing IS's main Syrian stronghold Raqa.
The main Kurdish component of the SDF, known as the YPG, or People's Protection Units, is the target of a military operation in northern Syria by US ally Turkey, which considers the YPG a "terrorist" group.
The attack on the SDF in eastern Syria could spell further trouble for the US-backed forces, which have largely avoided confrontation with pro-Assad units until now.