U.S. Army Maj. Gen. Jamie Jarrord, left, thanks a local military commander, Muhammed Abu Adeel, during a visit to a small outpost near the town of Manbij in northern Syria on Feb. 7 (Susannah George/AP)

Pro-government forces in Syria launched an assault on a base where U.S. troops were operating in support of local partner units, triggering a counterattack from U.S. aircraft, the U.S. military said on Wednesday.

In a statement, the U.S.-led military coalition tasked with battling the Islamic State called the attack southeast of the city of Deir Ezzour in eastern Syria “unprovoked.”

“The coalition remains committed to focusing on the defeat-Daesh mission in the Middle Euphrates River Valley and asserts its non-negotiable right to act in self-defense,” the statement said, using an Arabic term for the Islamic State.

The incident marks one of a limited number of times in which the United States has struck out directly at forces loyal to Syrian President Bashar al-Assad. Last year, U.S. aircraft shot down a Syrian jet after pro-government forces attacked U.S. allied units in northern Syria.

There are about 2,000 American troops in Syria, mostly operating in support of the Syrian Democratic Force (SDF), a Kurdish-dominated group that has helped expel the Islamic State from most of the areas it once controlled.

It comes as the United States seeks to close out its campaign against the extremists in Syria and Iraq. But the Trump administration’s hope for a lessening of hostilities in Syria’s larger civil war has not materialized so far, as Turkish, Russian, Iranian and other forces are brought into ever-closer proximity as they vie for control and pursue their own long-term interests in Syria.

A military official, who provided information about the attack on the condition of anonymity because he was not authorized to discuss it, said the attacking forces initiated the assault by lobbing artillery at the SDF and U.S. forces, which were located eight miles east of the Euphrates River. The river has acted as a “deconfliction line” between Syrian government forces and U.S.-backed SDF units in some areas of Syria.

On Jan. 17, Secretary of State Rex Tillerson outlined a range of U.S. policy goals in the Syrian conflict, including an "enduring defeat" of "ISIS and al-Qaeda." (Department of State)

Overall, the pro-government force included about 500 people supported by rocket and mortar systems and Soviet-era tanks, the official said. Up to 30 artillery and tank rounds came within 500 yards of an SDF headquarters.

U.S. and Kurdish forces responded to that skirmish with air and artillery strikes, the official said.

The official said about 100 pro-government forces were killed in the counterattack. Officials declined to say whether the airstrikes were conducted with manned or unmanned aircraft.

Officials said no U.S. troops were injured or killed. One SDF member was injured. 

The official said it appeared the pro-government forces were trying to reassert government control of areas that the SDF troops had seized from the Islamic State last year and possibly to capture oil fields in the area.

U.S. officials said it was not immediately clear whether the attacking forces were part of the Syrian military or belonged to foreign militias or other units who fight in support of the government.

Another military official, who also spoke on the condition of anonymity, said not all the attacking forces were wearing uniforms, making it more difficult to identify them. “It’s not always readily apparent” what sort of forces are maneuvering on the ground in Syria’s crowded battlespace, he said.

Neither was it clear whether Russia, which is conducting its own air campaign in support of the Syrian government, may have had any role in the incident.