The U.S. on Thursday shot down an armed drone and bombed pro-Assad forces on the ground who the U.S. says have violated a well-established de-confliction zone in southern Syria, a U.S. military spokesman said Thursday.

It’s the third time the U.S. has bombed the pro-regime fighters, a platoon-sized force that has taken up a position inside the U.S. safe zone, near the al-Tanf garrison where the U.S. has been training counter-Islamic State forces for months.