US warplanes have struck Syrian pro-regime forces in a mission to protect British and American soldiers in the country's southeast.
The airstrikes, which killed Syrian government forces and Iranian-backed militia, occurred close to the border with Iraq.
In a statement on Twitter, Operation Inherent Resolve said: "Coalition struck Syrian pro-regime forces advancing in a de-confliction zone near al Tanf posing a threat to US partner forces.
"This was despite Russian attempts to dissuade pro-regime movement towards al Tanf, coalition aircraft show of force and warning shots.
"Coalition forces have operated in the al Tanf area for many months training and advising vetted partner forces who are fighting ISIS."
Several regime tanks and trucks were reportedly destroyed.
The news that Russia was warned and was still unable to stop the convoy will call into question how much influence their commanders have over Assad and Iranian forces in parts of Syria.
US and UK special forces have been training Syrian rebels at a heavily fortified base in al Tanf.
Islamic State attacked the base last month and recently regime forces have been pushing forward nearby, raising fears of a collision with coalition forces.
A 34-mile defensive zone exists around the base, and it is believed these forces strayed inside that.
Iranian troops are said to be commanding the pro-Assad forces and a number of Lebanese Hezbollah fighters.
Exact casualties figures have not been provided, although some reports estimate 50 were killed.
A US official described the latest action as defensive and said it will not change US strategy in Syria.