American forces engaged Pro-Assad troops with artillery, rockets and air strikes on Tuesday following what CENTCOM has characterized as an “unprovoked attack” against U.S. backed forces in Syria. According to a statement released by Operation Inherent Resolve, American advisers were present at the time of the attack, though none were injured or killed.

“Syrian pro-regime forces initiated an unprovoked attack against well-established Syrian Democratic Forces headquarters Feb. 7,” the statement reads. “Coalition service members in an advise, assist, and accompany capacity were co-located with SDF partners during the attack eight kilometers east of the agreed-upon Euphrates River de-confliction line.”

According to the Syrian Observatory for Human Rights, the Syrian military’s attack against the Syrian Democratic Forces (SDF) headquarters killed 21 and injured 125, prompting a response from American assets in the region.

“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 CENTCOM release said.

According to reports from the region, American artillery, tanks and rockets engaged the pro-Assad forces that are believed to have been attempting to take territory captured from ISIS by the SDF. CENTCOM reports that no Americans were injured as they engaged roughly 500 attacking troops, as well as tanks and artillery. Assad’s troops did not fair as well, with more than 100 reported dead or injured.

A U.S. military official explained that the pro-Assad forces were “likely seeking to seize oilfields in Khusham that had been a major source of revenue for [ISIS] from 2014 to 2017.”

Russia and Syria both issued official statements immediately after the encounter accusing the United States of being in the nation illegally, and claiming the U.S. military action was taken in support of terrorists. The U.S. has long supported the Syrian Democratic Forces, often characterizing the group as “moderate,” and crediting them for playing a significant role in the defeat of ISIS in Syria. Bashar al Assad often refers to any group that stands in opposition to his formal government as terrorists, as the nation has been embroiled not only in a fight against the Islamic State, but an ongoing civil war.

“The recent incident once again shows that the United States’ illegal military presence in Syria is actually aimed at taking control of the country’s economic assets and not at fighting against the ISIS international terror group,” said a statement issued by the Russian Defense Ministry.