The US-led coalition fighting the so-called Islamic State has said it hit pro-Syrian government forces near the border with Jordan that "posed a threat" to American forces and rebel groups in the area.
The coalition hit the pro-Syrian government forces after warning shots, a show of force and apparent Russian attempts failed to dissuade them from moving into the "de-conflicted area", a coalition statement said.
Speaking to reporters, US Defence Secretary Jim Mattis said the US will defend its troops in case of "aggressive" steps against them. He said:
"We are not increasing our role in the Syrian civil war, but we will defend our troops."
"And that is a coalition element made up of more than just US troops, and so we will defend ourselves [if] people take aggressive steps against us."
The de-conflicted zone, agreed between the US and Russia, is located around Tanf, where the borders of Jordan, Syria and Iraq meet.
The coalition statement said its forces have been operating in the area "for many months training and advising vetted partner forces" in the battle against IS.
Syrian activists said the strike destroyed vehicles and left casualties.
It was the first such close confrontation between US forces and fighters backing President Bashar Assad.
The attack marks a new approach in what has become an intensely crowded and complicated war zone.
It was the coalition's first on pro-Assad forces in the battlefield, with military operations so far focused on Islamic State militants and Al-Qaeda-linked groups.