A U.S. F-15 fighter aircraft shot down an Iranian-made drone over southern Syria on Tuesday, a U.S. defense official said, marking the second time in just as many days that the U.S. military has taken a pro-Syrian government aircraft out of the sky.

The official, who spoke on the condition of anonymity because the incident had yet to be made public, said that the drone was in the same area where U.S. aircraft had shot down the same type of unmanned aircraft – an Iranian Shahed 129 – earlier this month. The drone, roughly the size of a U.S. Predator unmanned aerial vehicle, was armed and within range of striking U.S.-led coalition troops and their Syrian allies, the official said, who said the strike was in “self-defense.”

CNN was first to report on the incident.

The shoot-down is just the latest escalation between the United States and forces loyal to Syrian President Bashar Assad. In just over a month, U.S. forces have struck Iranian-backed Shia militias in southern Syria three times, and on Sunday a U.S. F/A-18 shot down a Syrian Arab Air Force Su-22 jet.

All of the strikes, the Pentagon has said, were in self-defense and legal under the Authorization for Use of Military Force passed after the Sept. 11, 2001 attacks.

The presence of Iranian-backed troops, artillery and tanks and U.S.-led coalition forces in southern Syria, namely around the At Tanf crossing at the Iraq-Syria border, has slowly turned the area into a flash point. The U.S. military has established a roughly 50-kilometer “deconfliction” ring around Tanf, warning the pro-Assad Iranian-backed forces through a Russian deconfliction channel that movement within the zone could be considered hostile. The Iranian drone that was shot down Tuesday was outside of the deconfliction area, the official said.

Despite bouts of relative calm, the pro-Assad Iranian-backed forces have continued to encroach, at one point moving along the periphery of the deconfliction area to the Iraqi border and driving a wedge between Tanf and the Euphrates River Valley. With pro-Syrian government forces now between U.S.-backed fighters at Tanf and predominantly Islamic State-controlled areas any future offensive in the area will be difficult.

Louisa Loveluck in Beirut contributed to this report.

AP