U.S.-led coalition air and local ground forces in Syria have killed more than 100 pro-Syrian fighters after an "unprovoked attack" on the U.S.-allied Syrian Democratic Forces (SDF) headquarters, a U.S. official says.

"Syrian pro-regime forces included approximately 500 personnel in a large, dismounted formation supported by artillery, tanks, multiple-launch rocket systems, and mortars," the official said on February 7, speaking on condition of anonymity.

Earlier in the day, an official statement by the U.S.-led Operation Inherent Resolve said coalition advisers were with the SDF when its base was attacked 8 kilometers east of the Euphrates River de-confliction line in Deir al-Zor Province.

"In defense of coalition and partner forces, the coalition conducted strikes against attacking forces to repel the act of aggression against partners engaged" in the fight against Islamic State (IS) extremists, the statement said.

No U.S. personnel were injured in the attack by the pro-regime forces, unnamed officials separately told U.S. media. The official statement did not list the number of attackers or casualties among the pro-government forces.

A U.S. official told CNN that the attackers crossed the Euphrates armed with artillery and other weapons.

Syrian state media later on February 8 confirmed that dozens of fighters were killed in U.S.-led coalition strikes overnight but appeared to deny the victims were soldiers.

"In a new aggression and in an attempt to support terrorism, coalition forces targeted popular forces" in the eastern province of Deir al-Zor, state television said, in an apparent reference to paramilitary groups allied to the regime.

The Russian Defense Ministry said that the U.S. strike wounded 25 Syrian pro-government fighters it said were searching for militants near an oil refinery.

In a statement on February 8, the ministry said the pro-government fighters had failed to coordinate their actions with the Russian military before launching the search mission.

The incident "again showed that the U.S. is maintaining its illegal presence in Syria not to fight the Islamic State group, but to seize and hold Syrian economic assets," the ministry claimed.

U.S. and Russian officials have repeatedly raised questions about each other's motives in Syria.

Russia and Iran back Assad, while the United States and Turkey support differing rebel groups fighting the government.

Syrian government forces are active on the other side of the river around the city of Deir al-Zor.

The SDF is an alliance of Arab and Kurdish militias in northern and eastern Syria and with U.S. backing are fighting against the government of Syrian President Bashar al-Assad.

Violence continues to rage in Syria’s civil war, which has killed hundreds of thousands of people since it began with a crackdown on protests in 2011.

All sides are also fighting the remnants of IS extremists who have been driven out of most of the territory they claimed earlier in the fighting.

The United States maintains about 2,000 troops in Syria, mostly working with the estimated 50,000-strong SDF.

With reporting by Reuters, CNN, AP, and Zvezda