A USA official said on Thursday the attack near the town of At Tanf destroyed at least one tank and a bulldozer, while another official said they followed warning shots by United States aircraft meant to dissuade the fighters from advancing further.
Download our Expert Edition e-book, Cyber: Continuous Diagnostics and Mitigation. The area has been considered a de-conflicted zone under an agreement between the USA and Russian Federation earlier this month.
The statement said the USA claims that they have warned the forces from advancing before firing are completely rejected. "There's been a lot of talk about Astana lately, but this is actually the main show for solving the Syrian conflict", the diplomat said.
The Pentagon did not immediately respond to requests for comment.
The US military carried out the strike on the militia as they headed towards the al-Tanf military base in southern Syria - near the Syria-Iraq-Jordan border - used by US and US-backed rebel forces.
The reports indicated that the military convoy hit consisted of members of a pro-Assad Syrian militia, not the official Syrian armed forces.
The coalition said the de-confliction zone "remains in effect".
The official described Thursday's strike as a defensive action that does not change the US policy of focusing military efforts on defeating ISIS.
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The officials weren't authorized to speak publicly on the matter and demanded anonymity.
"In every meeting we remind the attendees that there is terrorism by terrorists and also terrorism by states committed against our country", al-Ja'afari said. That battle has already caused clashes between government forces and Syrian rebels and raised concern of pro-government militias making a bid for controlling the border with Iraq.
The parliamentarian, however, wondered why pro-government forces which are advancing on their own territory should be caught in an attack.
In the end, USA fighters were forced to engage the Syrian force by launching airstrikes against the convoy.
-Sarah El Deeb in Beirut and Lolita Baldor in Washington.
Syria's ally Russia said earlier today that the strike had hit civilians and was unacceptable, Russian news agencies reported. The coalition had so far kept its military operations focused on Islamic State militants and al-Qaida-linked groups.
Coalition forces have operated in the al Tanf area for many months training and advising vetted partner forces who are fighting ISIS.
The US said Russian Federation was told in advance and that warning shots were ignored.
But Syria's opposition delegation to Geneva lauded the airstrike, saying it helps curb Iran's influence in his country.