More than 100 fighters aligned with Syrian President Bashar al-Assad were killed overnight when U.S. coalition and coalition-backed local forces repelled their attack in eastern Syria, a U.S. official said on Thursday.
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. The official spoke to Reuters on condition of anonymity.
In reports that appeared to confirm the official's comments, Syrian state media said the U.S.-led coalition against Islamic State had bombed pro-government forces east of the Euphrates river in Deir al-Zor province. One of the state TV outlets reported "dozens of dead and wounded".
No American troops were killed or wounded in the incident, officials said.
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.
One SDF fighter was wounded, the official said.
"We suspect Syrian pro-regime forces were attempting to seize 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."
Neither U.S. officials nor the U.S.-backed coalition have offered details on the identity of attacking forces. The Syrian army is supported by Iranian-backed militias and Russian forces.
Syrian state media said on Thursday the U.S.-led coalition had bombed "popular forces fighting (Islamic State) and the (Syrian Democratic Forces) east of the Euphrates river" in Deir al-Zor, causing casualties.
Statements carried by Syrian television stations called it a "new aggression" and "an attempt to support terrorism".
A reporter for the state TV station Ikhbariya said there were "dozens of dead and wounded" as a result of the attacks and described the groups it said had been bombed by the U.S.-led coalition as "local people fighting (Islamic State) and the SDF".
The U.S.-led coalition had alerted Russian officials about the presence of SDF forces in the area far in advance of the thwarted attack, the U.S. official said.
"Coalition officials were in regular communication with Russian counterparts before, during and after the thwarted (enemy) attack," the official said.
The coalition said the attack occurred around 8 kilometres (5 miles) "east of the Euphrates River de-confliction line in Khusham", a town southeast of the provincial capital Deir al-Zor city.