The US-led coalition fighting ISIS in Syria conducted air and artillery strikes against pro-regime forces in Syria on Wednesday, killing over 100 pro-regime fighters, the coalition said in a statement.
The coalition said the strikes had been carried out after forces allied with Syrian President Bashar al-Assad "initiated an unprovoked attack" against a well-established Syrian Democratic Forces headquarters where coalition advisers were working with US-backed Syrian fighters.
Some 500 pro-regime troops carried out the attack using artillery, mortar fire and Russian-made tanks, a US military official said.
"Syrian pro-regime forces maneuvered T-54 and T-72 main battle tanks with supporting mortar fire in what appears to be a coordinated attack on Syrian Democratic Forces approximately 8 kilometers east of the Euphrates River de-confliction line in Khusham, Syria," the military official said.
The official said that about "20 to 30 artillery and tank rounds landed within 500 meters" of the Syrian Democratic Forces headquarters before the US-backed fighters and coalition forces "targeted the aggressors with a combination of air and artillery strikes."
"We estimate more than 100 Syrian pro-regime forces were killed while engaging (Syrian Democratic Forces) and Coalition forces," the official said.
"This action was taken in self-defense," he said, adding that pro-regime forces that returned across the river had not been targeted.
The official said no coalition or US personnel had been killed or wounded in the engagement but that one member of the Syrian Democratic Forces was wounded.
He said the coalition suspected that pro-regime forces were attempting to seize territory, including lucrative oil fields, that the Syrian Democratic Forces had liberated from ISIS in September.
They "were likely seeking to seize oilfields in Khusham that had been a major source of revenue for Daesh from 2014 to 2017," the official said, using the Arabic acronym for ISIS.
Another US official who spoke to CNN said it was unclear exactly who the pro-regime forces were but that the US is looking into whether Russian contractors operating in the vicinity might have been involved. This official added that there is no direct evidence yet that the Russians had fired on the Syrian Democratic Forces facility where the US advisers were located. They have not ruled out that Iranian backed-forces might have also been involved in the attack, this official said.
The coalition "cannot speculate on the exact makeup of the Syrian pro-regime forces who conducted the attack at this time," the US military official said.
The coalition said the attack took place in the Middle Euphrates River Valley, five miles east of the agreed-on "de-confliction" line designed to separate Russian-backed regime troops and US-backed forces in Syria.
The US military official said the coalition had informed Russia of the presence of pro-regime forces in the area before the attack via a hotline established as part of the de-confliction line, adding that the coalition had observed a buildup of pro-regime troops over the past week.
"Coalition officials were in regular communication with Russian counterparts before, during and after the thwarted (Pro-Regime Forces) attack," the military official said. "Russian officials assured Coalition officials they would not engage Coalition forces in the vicinity."
The US maintains about 2,000 troops in Syria, who mostly work with the over 50,000-strong Syrian Democratic Forces, a mix of Kurdish and Arab fighters who have been the primary US-backed force fighting ISIS in Syria.
This is not the first time the US military has clashed with pro-regime forces in Syria.
A US Navy F/A-18 shot down a Syrian regime Su-22 jet last June that had attacked US-backed forces on the ground. Coalition aircraft have struck regime forces who were perceived as posing a threat to a base at At Tanf, Syria, that houses US and coalition military advisers. The US also shot down a pro-regime drone after it dropped a munition in the vicinity of US troops in Syria.