The Pentagon says U.S. commanders notified Russian counterparts of an air and ground assault on pro-Syrian fighters in response to an attack on U.S.-allied militias in eastern Syria.
Defense Department spokeswoman Dana White on February 8 declined to say whether there were any Russian soldiers or mercenaries among the dozens killed in the attack, which occurred just east of the Euphrates River deconfliction line in Deir al-Zor Province.
The U.S.-led Operation Inherent Resolve said in a statement earlier that coalition advisers were with the Syrian Democratic Forces when their base was attacked on February 7.
"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.
White said pro-regime forces lobbed 20-30 tank rounds within 500 meters of the base.
"Our forces have the inherent right of self-defense," she said. "We are not looking for a conflict with the [Syrian] regime."
U.S. Defense Secretary Jim Mattis later called the attack "perplexing," adding that initial information did not reveal any Russian forces among the attackers.
"We don’t think there are any Russians, but I am not confirming that," he told reporters.
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.
In a letter addressed to UN Secretary-General Antonio Guterres, Syria’s Foreign Ministry said the attack "represents a war crime and a crime against humanity."
It also accused the United States of using "the excuse of fighting terrorism to set up illegitimate bases on Syrian territory."
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 ministry also appeared to put some of the blame on pro-government fighters, saying in a statement that they 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.
'Regrettable' Strike Says Russia
In New York, Russia's United Nations envoy described the coalition strike as "regrettable," and said he would raise the matter during a Security Council briefing on the Syria's humanitarian situation later on February 8.
"That's very regrettable, we will raise that issue, we will ask them what happened," Ambassador Vassily Nebenzia told reporters.
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.