Pro-Syrian forces hit in strike 'posed a threat'
The US-led coalition fighting Islamic State has said it hit pro-Syrian government forces near the border with Jordan that "posed a threat" to US 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 those troops from moving into the "de-conflicted area", a coalition statement said.
The de-conflicted zone, agreed between the US and Russia, is located around Tanf, where the borders of Jordan, Syria and Iraq meet.
The statement said coalition 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.
Speaking to reporters, US Defence Secretary Jim Mattis said the US will defend its troops in case of "aggressive" steps against them.
He was asked if the airstrike increases the US role in the Syrian war.
"We are not increasing our role in the Syrian civil war, but we will defend our troops," Mr Mattis said.
"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 US strike marks a new approach in what has become an intensely crowded and complicated war zone.
Thursday's strike was the coalition's first on pro-Assad forces in the battlefield. The coalition had so far kept its military operations focused on Islamic State militants and al Qaida-linked groups.