A U.S. airstrike in Syria targeted pro-regime forces who were threatening a coalition base where advisers train anti-Islamic State fighters, according to a U.S. military official.

The forces came within a 34-mile defensive zone around the al-Tanf base in southern Syria, according to the official who was briefed on the action but not authorized to discuss the incident publicly.

U.S. military officials have not yet determined if Syrian army forces were targeted in the strike or if they were militias aligned with the government of Syrian President Bashar Assad.

The airstrike targeted a tank and two earth movers that were building fighting positions within the defensive zone, the military official said.

The U.S.-led coalition attempted to use a special hotline they have established with the Russians to warn the Syrian government to remove the forces from near the coalition base. The Russians relayed the message to the Syrian government, but the forces did not withdraw, the official said.

The U.S.-led coalition is battling the Islamic State, also known as ISIS, in Syria and is generally not attacking Assad. An exception was the cruise missile strike in retaliation for Assad’s use of chemical weapons.