The American-led coalition in Syria conducted an airstrike on pro-regime forces after an "unprovoked" attack on the headquarters of the Syrian Democratic Forces Wednesday.

Operation Inherent Resolve stated the attack from the pro-regime groups on the SDF was unprovoked, and in response, the coalition, led by the U.S., conducted strikes against the pro-Assad attackers, operation officials stated Wednesday.

"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 Global Coalition's defeat-Daesh mission," the statement read.

Coalition service members were located alongside the Syrian Democratic Forces when the attack occurred several kilometers from the predetermined de-confliction line.

According to NBC, no U.S. casualties have been reported and a battle damage assessment has been initiated.

It's uncertain who the pro-regime forces involved were, but the U.S. is seeking details to determine if Russian contractors operating nearby were involved, although there's no direct evidence that indicates they were involved, a U.S. official told CNN. Additionally, it's possible Iranian backed-forces participated, the official added.

Military officials said the coalition is determined to defeat the Islamic State of Iraq and Syria in the area and that actions of self-defense are a “non-negotiable” right.

The White House referred questions back to the Defense Department when reached by the Washington Examiner.

This is a developing story. Check back to the Washington Examiner for more information as it becomes available.