The U.S.-led coalition and its local allies in Syria struck pro-government forces with deadly air and artillery fire overnight to repel "an unprovoked attack" near the Euphrates, the coalition said on Thursday.
The incident underscores the potential for further conflict in Syria's oil-rich east, where the U.S.-backed Syrian Democratic Forces (SDF) alliance of Kurdish and Arab militias holds swathes of land after its offensive against Islamic State (IS) militants.
Syrian President Bashar al-Assad, who is supported by Russia and by Shi'ite militias backed by Iran, has said he wants to take back every inch of Syria.
The pro-government forces were "likely seeking to seize oilfields in Khusham" east of the Euphrates in Deir al-Zor province, said a U.S. official speaking on condition of anonymity.
The attack was carried out by 500 troops backed by artillery, tanks, multiple-launch rocket systems and mortars but the coalition and its local allies killed more than 100 of them, the official said.
Syrian state television reported that the coalition had caused "dozens of dead and wounded" by bombing pro-government forces. But a commander in the military alliance supporting Assad disputed the death toll, saying seven pro-government forces were killed and 27 injured.
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 United States and Russia maintain regular contacts in eastern Syria to prevent unexpected confrontation between the forces they support there.
Russia's Defence Ministry said the pro-government militias involved in the incident had been carrying out reconnaissance and their activities had not been previously agreed with Russia.
No American troops were killed or wounded in the incident, officials said.
Some U.S. troops had been embedded at the time with the SDF, whose headquarters in Deir al-Zor province had been a target of the attack.
One SDF fighter was wounded, the official said. Nouri Mahmoud, spokesman for the SDF's most powerful element, the Kurdish YPG militia, described the clash as "skirmishes" and said each side had returned to their former positions.
"We suspect Syrian pro-regime forces were attempting to seize terrain SDF had liberated from Daesh in September 2017," the U.S. official said.
Neither U.S. officials nor the U.S.-backed coalition have offered details on the identity of attacking forces.
The coalition said in an email the pro-government forces had initiated hostilities with artillery fire, tank manoeuvres and mortar fire after a steady buildup of forces over the past week.
A reporter for Syrian state TV station Ikhbariya described the groups it said had been bombed by the U.S.-led coalition as "local people fighting (IS) and the SDF".
Russia's Interfax cited the Defence Ministry as saying the incident showed the U.S. goal in Syria was not to battle IS but "the capture and withholding of the economic assets", an apparent reference to the Khusham oil field.
Russian commanders held talks with coalition representatives after the incident, it added.
The coalition said the attack occurred around 8 km (5 miles) "east of the Euphrates River de-confliction line in Khusham", a town southeast of the provincial capital Deir al-Zor city.