A U.S. airstrike in Syria Thursday 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 official said. The official did not provide an estimate of casualties.

The U.S.-led coalition attempted to use a 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 U.S. cruise missile strike that were launched last month in retaliation for Assad’s chemical weapons attack that killed 86 people.

Several hundred U.S. advisers are in Syria to help train and organize a force to defeat ISIS. The U.S.-led coalition is also supporting the force with airstrikes.

Aside from the April cruise missile strike, the coalition-backed forces have avoided contact with Assad regime forces.

The official described Thursday's strike as a defensive action that does not change the U.S. policy of focusing military efforts on defeating ISIS.