U.S. forces targeted the pro-government troops with airstrikes and artillery after they launched an attack against a base belonging to the U.S.-backed Syrian Democratic Forces in the eastern province of Deir al-Zour, according to military spokesman Col. Thomas F. Veale.
U.S. military personnel advising the Kurdish-led SDF were at the base at the time, he said.
Syria’s official news agency SANA said the attack left “scores of persons dead and others injured.” A U.S. military official who spoke on condition of anonymity had earlier said the U.S. strikes killed around 100 troops, but Veale said the military does not have an exact figure for the casualties.
One SDF fighter was injured and there were no U.S. or coalition casualties, he added.
For its part, the British-based Syrian Observatory for Human Rights reported that 45 members of the pro-government force were killed in the strike.
The clash was the biggest confrontation yet between the U.S. military and the pro-Syrian government alliance that includes Russia and Iran, since U.S. troops began deploying in Syria in 2015 in support of local Kurdish and Arab fighters taking on the Islamic State.
It coincides with heightened tensions between the United States and the Syrian government as the Islamic State war winds down, leaving eastern Syria roughly partitioned between U.S.-backed forces and the Russian-backed Syrian government.
The Syrian government has repeatedly called for U.S. troops to leave Syria now that the ISIS war is over, and has regularly threatened to push them out by force.
The Trump administration last month, however, announced a new Syria strategy that envisages U.S. troops remaining in eastern Syria until there is a peace settlement to the wider war that includes a transition away from the rule of President Bashar al-Assad.
A deconfliction line establishing the Euphrates River as the divide between the rival forces was negotiated between the U.S. and Russian militaries last year, and until now it has served to mostly keep the armies apart.
Channels of communication between the United States and Russia remained open throughout the Thursday night clash, Veale said, and the United States informed Russia ahead of the strike.
The hostilities erupted, he said, after a battalion-sized detachment of infantry — several hundred fighters — backed by tanks and artillery began advancing on a SDF position in the town of Khisham, located around five miles east of the informal deconfliction line established between the U.S. and Russian militaries last year.
When the advancing forces unleashed a volley of 20-30 tank and artillery rounds toward the base, U.S. warplanes and artillery struck back, he said.
Veale said the U.S. military does not know the composition of the pro-regime force, which could have included foreign Shiite militias drawn from countries such as Iraq, Lebanon and Afghanistan that have been armed and trained by Iran to support the depleted Syrian army.
The SANA report described the troops involved in the clash as “popular forces,” suggesting they were irregular militias.
The clash came after a week in which the U.S. military had noted a build up of pro-government forces in the vicinity, and it appeared the advancing forces were intending to seize nearby oil fields that the SDF had captured from the Islamic State last year, Veale added.