US aircraft shot down an Iranian-made drone that fired on coalition forces patrolling with partner forces in southern Syria, a coalition spokesman told reporters on Thursday.

This is the first time that pro-Syrian regime forces — which the US says includes Iranian-backed Shia militias — have fired on the US-led coalition.

A US official told CNN that a US F-15 jet shot down the Iranian-made Shahed 129 drone, which was being controlled from a location in Syria north of At Tanf.

It was shot down after it dropped one of several weapons it was carrying near a position where coalition personnel are training and advising partner ground forces in the fight against ISIS.

The “munition did not have an effect on coalition forces,” according to spokesman Col. Ryan Dillon.

“There are still some pro-regime forces that remain inside the de-confliction zone,” said Dillon adding, “We remain ready to defend ourselves against any threat.”

Dillon indicated the US wanted to focus on fighting ISIS rather than these forces.

A US defense official said that, for now, this was a proportional response. The munition dropped by the drone was a “dud” and is being perceived right now as a “show of force” by those who fired it.

The incident marks an escalation in the area around Al-Tanf — a base on the Syrian-Jordanian border manned by US and British Special Forces that are advising an anti-ISIS Syrian rebel group known as Maghawir al-Thawra, or the Commandos of the Revolution.

The US-led anti-ISIS coalition also conducted another airstrike Thursday against pro-regime forces in the de-confliction zone around the coalition base, two US officials told CNN.

The strike took place after three technical vehicles were observed crossing into the restricted area. Two of the vehicles were hit about 24 miles from the At Tanf base.

This was the third coalition strike in recent weeks against pro-regime forces near At Tanf and it comes days after a highly unusual move from the coalition where they permitted a pro-regime Syrian aircraft to conduct an airstrike inside a restricted zone in southern Syria where American forces were stationed.

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SOURCE: CNN, Barbara Starr, Ryan Browne and Zachary Cohen