The U.S. -led coalition said it struck pro-Syrian government forces that were "advancing well inside an established de-confliction zone" in southern Syria on Thursday.
"This action was taken after apparent Russian attempts to dissuade Syrian pro-regime movement south toward At Tanf were unsuccessful, a coalition aircraft show of force, and the firing of warning shots", it said in a statement. When the forces did not respond, the commander called for an airstrike. "All to no avail", the official said.
The strike was near a base in An Tanf, which is used by the coalition to train vetted Syrian opposition to help fight against Isis.
The US defence official told AFP that the strike did not signal a strategic shift for the coalition, which has focused on battling IS and has tried to avoid intervening in Syria's civil war. There were USA advisers operating at the base at the time of the airstrikes.
"They were potentially probing to see how close they could get to al Tanf", an anonymous U.S. official said.
Defense Secretary Jim Mattis told reporters at the Pentagon that the US strike was defensive in nature.More news: Talansky climbs to Stage 5 win in Tour of California
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Pro-Assad Shia militias backed by Iran have historically operated in that area.
A Western intelligence official, speaking on condition of anonymity, has said the strike sent a strong message to Iranian-backed militias that they would not be allowed to reach the Iraq border from Syria.
The U.S. -led coalition did not signal it would cede ground around Tanf.
In the end, USA fighters were forced to engage the Syrian force by launching airstrikes against the convoy.
The statement by the USA -led coalition acknowledged a zone but did not offer any details about it, other than to say it was still active. The last encounter between the groups occurred last month when Syrian troops passed near a special operations group in the Kurdish areas, but that remained peaceful.
Unlike the USA strike against the al-Shayrat air base in March, which the Trump administration ordered in retaliation for a regime chemical weapons attack, Thursday's strike seemed created to protect the United States forces based at Tanf.