More than 100 fighters aligned with Syrian President Bashar al-Assad were killed after U.S.-led coalition and coalition-backed local forces thwarted a large, apparently coordinated attack between Wednesday and Thursday, a U.S. official said.

The heavy death toll underscored the large size of the attack, which the U.S. official said included about 500 opposing forces, backed by artillery, tanks, multiple-launch rocket systems and mortars.

He said no American troops were killed or wounded in the incident but one SDF fighter was wounded.

Still, some U.S. troops had been embedded at the time with Syrian Democratic Forces (SDF), whose headquarters in Syria’s Deir al-Zor province had been a target of the attack.

“We suspect Syrian pro-regime forces were attempting to seize the terrain SDF had liberated from Daesh in September 2017,” the official said.

The forces 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.

Neither U.S. officials nor the U.S.-backed coalition have offered details on the attacking forces.

The Syrian army is supported by Iranian-backed militias and Russian forces. (Xinhua/NAN)