Washington(CNN) US-led coalition aircraft bombed pro-Syrian regime forces Thursday that "were advancing well inside an established de-confliction zone," according to a statement issued by Operation Inherent Resolve, the official name for the coalition fighting ISIS.
The statement said the pro-regime militias "posed a threat to US and partner forces at At Tanf," a remote base in southern Syria near the Syria-Jordan-Iraq border that coalition advisers have used to train "Vetted Syrian Opposition" to aid the fight against ISIS.
Two US aircraft were dispatched as a "show of force" to get the Syrian vehicles to turn around, according to a US official.
The official statement said that the vehicles did not stop despite the show of force and warning shots.
A second US defense official said an airstrike hit the convoy after the vehicles continued toward the base, striking an enemy tank and bulldozer, or front loader-type vehicle.
The airstrikes were considered to be carried out in "self-defense" given the militias' proximity to a base used by elite US and coalition troops. Such deliberate airstrikes against pro-regime forces are extremely rare, with the last such strike being carried out in April in retaliation for the use of chemical weapons.
Pro-Assad Shia militias backed by Iran have historically operated in that area.
A convoy of 20 pro-regime vehicles were headed toward At Tanf Wednesday night, searching vehicles and buildings along the way, according to a US official. Thirteen of the vehicles breached the "de-confliction zone" around the base that is the area that the coalition has communicated to the Russians and their regime alliesto stay away from. The official said five of the vehicles came within 29 kilometers of the base at about midnight EST Thursday when the aircraft were dispatched.
The official statement said that Russia had made "apparent" attempts to "dissuade Syrian pro-regime movement south toward At Tanf," but that those efforts were "unsuccessful."
There is a Syrian regime base nearby, the official said, adding that forces there are equipping to handle about a battalion's worth of troops.