"Syrian pro-regime forces included approximately 500 personnel in a large, dismounted formation supported by artillery, tanks, multiple-launch rocket systems, and mortars", a USA military official said, according to Radio Free Europe.
"We estimate more than 100 Syrian pro-regime forces were killed while engaging (Syrian Democratic Forces) and Coalition forces", the official said.
One Syrian rebel soldier was wounded but no USA forces were killed or wounded, according to NPR's Tom Bowman, who is reporting from elsewhere in Syria.
The coalition "cannot speculate on the exact makeup of the Syrian pro-regime forces who conducted the attack at this time", the U.S. military official said.
A rare USA airstrike in Syria has killed approximately 100 pro-government troops. One official added that Russian contractors may have been involved in the initial attack. "There are about 2,000 US troops in Syria focused on the fight against ISIS", said Tom.
Syria blasts U.S. accusations on chemical weapons use as 'lies'
The activist-run Ghouta Media Center said at least two, Douma and Harasta, was struck by ground-to-ground missiles. It said one of its workers was also killed by an airstrike while responding to a call for rescue in the area.
The Euphrates, which flows through northeastern Syria, functions as a demarcation line separating the American-backed SDF and forces of the Bashar Assad regime backed by Russian Federation and Iran who are active around the city of Deir el-Zour. This official added that there is no direct evidence yet that the Russians had fired on the Syrian Democratic Forces facility where the United States advisers were located.
US and Kurdish forces responded to that skirmish with air and artillery strikes, the official said.
Officials said it appeared that the pro-regime forces were trying to take back land that the SDF had liberated from IS fighters last September, including oil fields in Khusham.
On May 18, they attacked pro-regime forces that continued approaching coalition troops despite warning shots and Russian attempts to halt the troops.
The SDF is an alliance of Arab and Kurdish militias in northern and eastern Syria and with US backing are fighting against the government of Syrian President Bashar al-Assad.