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.

Col. Ryan Dillon, a U.S. military spokesman, said the drone was the same size and closely resembled a U.S. MQ-1 Predator, but he did not identify its country of origin or say where it took off from.

"The drone did fire on the coalition forces that were operating and conducting a patrol to the east, outside of the deconfliction zone, doing patrols with our partner forces, as we have done for many months now," Dillon told reporters at the Pentagon, via a video link from Baghdad.

No U.S., coalition or partner forces were hurt in the drone strike, which Dillon said "hit dirt."

But after dropping the munition near the coalition patrol, the unmanned aircraft was still armed and posed a threat, so it was shot down.

The downing of the drone followed an earlier incident this week in which U.S. warplanes struck the pro-regime forces on the ground, inside the 35-mile buffer zone, which were judged to be posing an imminent threat to the training base at the al-Tanf border crossing.

Two "technical vehicles," pickup trucks with weapons, were hit, according to Dillon.

"The coalition does not seek to fight Syrian regime or pro-regime forces, however we remain ready to defend ourselves against any threat," Dillon said. "Our focus is on ISIS."

The small element of pro-regime forces – described as "platoon-size" – remains inside the zone in the northwest, while a larger force is gathered just outside the zone, its intentions unclear.

"The coalition has been very clear through the deconfliction line, through leaflets, messages and actions, to warn pro-regime forces not to advance toward, or otherwise threaten coalition forces at al-Tanf," Dillion said.

"The coalition calls on all parties in southern Syria to focus their efforts on the defeat of ISIS, which is our common enemy and the greatest threat to the region and the rest of the world."

Despite repeated warnings, there is no sign the forces inside the U.S. declared zone are moving.

"We will continue to attempt to get them to vacate that area," Dillon said.

This was the second bombing in the restricted area this week, and the third U.S. strike since the forces moved into the area in mid-May.