U.S. officials say American advisors accompanying Syrian opposition forces came under attack by government-backed troops in Deir el-Zour Province. U.S.-led coalition forces retaliated by launching airstrikes against the troops loyal to Syrian President Bashar Assad's regime, according to the Defense Department.
One Syrian rebel soldier was wounded but no U.S. forces were killed or wounded, according to NPR's Tom Bowman, who is reporting from elsewhere in Syria.
In a statement, the Pentagon said, "Syrian pro-regime forces initiated an unprovoked attack against well-established Syrian Democratic Forces headquarters today."
Officials say the airstrikes were in defense "of coalition and partner forces."
"We don't know who the pro-regime forces are, whether they're independent militias or part of the Syrian government. Attacks on U.S. troops by pro-regime forces have been unusual, as are U.S.-led air strikes against pro-regime forces. There are about 2,000 U.S. troops in Syria focused on the fight against ISIS," said Tom.
Citing U.S. military sources, he reports that the coalition had been observing a slow buildup of pro-regime forces over the past week and alerted Russian officials of their presence in Kusham, Syria, via the "de-confliction" phone line "well in advance of the attack."
Tom says the military sources suspect the pro-regime forces' attack is part of a plan to seize oil fields the rebels had taken from ISIS in 2017.
Officials say the U.S. was in contact with Russian counterparts during and after the attack and were assured that Russia would not engage coalition forces in the vicinity.
U.S. officials say about 500 pro-Assad regime forces participated in the attack on the coalition location, using heavy artillery, T-54 and T-72 main battle tanks and mortar fire.