US-led coalition aircraft struck a pro-regime convoy apparently transporting Shia militia men in Syria as it headed toward a remote coalition garrison near the Jordanian border last night, US officials said.

While officials stressed the defensive strike did not signal deepening US involvement in Syria's civil war, it nonetheless raises questions about the ongoing feasibility for the coalition to maintain its singular focus on the so-called Islamic State group.

In a statement, the coalition said the strike had occurred "well inside" an established de-confliction zone northwest of the At-Tanf garrison, where British and US commandos have been training and advising local forces fighting IS.

"A convoy going down the road didn't respond to numerous ways for it to be warned off from getting too close to coalition forces in At-Tanf," a US defence official told AFP on condition of anonymity.

"Then there was finally a strike against the lead portion of that movement."

A second defence official told AFP the forces in the convoy appeared to have been Shia militia.

Attempts to stop the convoy included a call to the Russians, who are working with the Syrian regime, then a "show of force" in the skies above the vehicles, followed by warning shots.

The first official said the convoy was "significant" in size, but only the lead vehicles were hit.

Coalition officials said Russian forces had apparently unsuccessfully tried to dissuade the pro-regime movement south.

Later, Secretary of Defence Jim Mattis stressed that US is not getting more involved in Syria's civil war.

"We are not increasing our role in the Syrian civil war, but we will defend our troops," Mattis said.

"We will defend ourselves (if) people take aggressive steps against us. And that's been a going-in policy of ours for a long time."

Syria has condemned the strike as a "brazen attack" that undermined claims the alliance is fighting extremists.

"This brazen attack by the so-called international coalition exposes the falseness of its claims to be fighting terrorism," "a military source told state media..