Maghaweer al-Thawra, a U.S.-backed anti-government Syrian fighter, stands on a vehicle with a machine gun, next to an American soldier who also stands on his armored vehicle as they take their position at the Syrian-Iraqi crossing border point of Tanf.
(Hammurabi’s Justice News via Associated Press)

A U.S. aircraft shot down a pro-Syrian government drone after it fired upon a group of U.S.-led coalition forces in southeastern Syria on Thursday, the Pentagon said.

Col. Ryan Dillon, spokesman for the U.S.-led coalition in Iraq and Syria, told reporters that no coalition forces were injured in the attack and that the drone’s munition struck “dirt.”

The incident marks a significant escalation around the Tanf border crossing, a vital link that connects Iraq and Syria. Iran considers the area — mostly made up of scrub and desert — part of an integral supply route that connects Tehran with Iraq, Lebanon and Syria. In the same area, U.S.-led Special Operations forces have been quietly training a small detachment of Syrian opposition fighters in anticipation of a broader campaign against the Islamic State in the Euphrates River Valley.

In recent weeks, however, pro-Syrian government forces — composed mostly of Iranian-backed Shiite militias — have steadily encroached on the small 200-strong garrison. In response, U.S. forces bolstered their defenses and conducted two strikes on the advancing government forces, one on May 18 and again Tuesday. The Pentagon labeled the airstrikes an act of self-defense after accusing the pro-Syrian government forces of entering a U.S.-established 34-mile wide deconfliction zone around the small base.

Dillon said that U.S. forces have communicated repeated warnings to the pro-government forces through a deconfliction line set up between the U.S. and Russian militaries in the region. Additionally, he said, U.S. aircraft have dropped thousands of leaflets over the contingents of government troops encamped in the desert around Tanf telling them to stop their advance and leave the area.

An alliance supporting Syrian President Bashar al-Assad said Wednesday that it would attack U.S.-led coalition forces in the area if the United States crossed unspecified “redlines.”

The attack on the drone, Dillon said, occurred outside the established deconfliction zone, where U.S.-led coalition troops were patrolling with their Syrian opposition counterparts. It is unclear whether the coalition forces were marked or easily identifiable. The pro-government forces gave no warning of the strike, Dillon said, though in the days leading up to the strike Iranian Hezbollah-associated Twitter accounts posted what appeared to a video taken from a drone, observing a U.S.-unmanned aircraft in the same area.

Dillon said the drone appeared to be in similar size to a U.S. MQ-1 Predator and was loading with additional munitions when it was shot down. Little is known about the Syrian government’s drone program, but Iranian unmanned aircraft, such as the Shahed 129, have routinely appeared in Syrian skies in the past several years. Dillon would not say whether the drone belonged to the Syrian government or Iran, only that it was “pro-regime.”

Thursday’s drone shoot-down came after U.S.-led aircraft struck two vehicles that belonged to pro-Syrian government forces that were advancing on the Tanf garrison earlier in the day.

Abu Waleed, a commander from Usoud al-Sharqiya, one of the opposition groups loosely partnered with the U.S.-led coalition’s effort in the area, said that a “convoy’” of pro-regime militia was heading toward a new coalition outpost located to the northeast of Tanf. Waleed said it was destroyed by the strikes.

It is unclear how many pro-Syrian government troops were killed or wounded in airstrikes and what legal rationale will adequately cover the U.S.-led coalition if it continues to strike Syrian forces operating within their sovereign borders.

Louisa Loveluck in Beirut contributed to this report.