Coalition officials initially tried to stem the advance of the Iranian convoy with a series of warning shots and US aircraft carrying out a low flyover above the convoy in a show of force.
US-led coalition aircraft Thursday struck a pro-regime convoy apparently transporting Shia militiamen in Syria as it headed toward a remote coalition garrison near the Jordanian border, U.S. defense officials said.
The Syrian government has said an aerial "aggression" on one of its military positions near the border with Jordan by the US-led coalition killed several soldiers.
In a separate statement, a "senior US defense official" told Fox News that "the coalition commander assessed the threat and after shows of force didn't stop the regime forces and those forces refused to move out of the deconfliction zone, the commander on the ground called for the air strike as a matter of force protection".
He was referring not only to Thursday's strike but also to others carried out by the US -led coalition in recent months that have claimed the lives of civilians in areas held by the Islamic State group.
Russian Foreign Minister Sergey Lavrov called the strike a violation of Syrian sovereignty and said he was "unaware" that Russia had been given any warning ahead of the strike.
Mattis said the convoy appeared to move into the area against the advice of Russian Federation.More news: Saudi Arab, US agree to bolster ties in different sector
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The statement added that the USA -led coalition forces will continue to defend themselves against IS or any other threat to the coalition or its allies in the area.
President Bashar Assad's army is trying to prevent US-backed forces from leading that fight.
It said "those who claim to be fighting terrorist groups, should not fire at the Syrian army because it's the legitimate force fighting the groups, but should direct their fire against the terrorist groups".
"In every meeting we remind the attendees that there is terrorism by terrorists and also terrorism by states committed against our country", Mr al-Ja'afari said.
Another US official said the jets carried out "shows of force" in the skies above the tank convoy and fired warning shots before bombing the lead vehicles, which were moving to within 27 kilometers of the garrison. Mattis said he believed the Iranian-directed forces moved into the zone against the advice of Russian Federation but that he was unable to confirm that with certainty.
In the first concrete results from talks this week on ending Syria's conflict, the United Nations said the warring sides had agreed to set up expert committees to discuss "constitutional issues".
Rebel forces have received backing from Gulf states, Turkey, and the West, while the Syrian army is bolstered by Russia, Iran, and fighters from neighbouring Lebanon and Iraq.
Earlier this month, they agreed to create four "de-escalation zones" across some of Syria's bloodiest battlegrounds, and observers said de Mistura would be scrambling to match Astana's momentum.