The US-led coalition says regime forces had "posed a threat" to US and partner forces at Tanf base near the joint Syria-Iraq-Jordan border. Russia says the strike violated Syria's sovereignty.
US-led coalition aircraft on Thursday struck a convoy of pro-Syrian regime forces that were "advancing well inside an established de-confliction zone" in southern Syria.
The coalition said the forces "posed a threat" to US and partner forces at Tanf base near the joint Syria-Iraq-Jordan border, where US special forces were operating and training Free Syrian Army rebels.
"This action was taken after apparent Russian attempts to dissuade Syrian pro-regime movement south towards At Tanf were unsuccessful, a coalition aircraft show of force, and the firing of warning shots," the coalition said in a statement.
Hampering fight against Daesh
The Syrian regime said the air strike hit without elaborating and hampered "its efforts to fight Daesh."
Syria's ally Russia on Friday condemned the strike, saying it hit civilians and was unacceptable, Russian news agencies reported.
Speaking in Geneva, Russian Deputy Foreign Minister Gennady Gatilov said the US strike had violated Syria's sovereignty and would not help efforts to find a political solution to the conflict.
The Tanf Garrison is used by US and British commandos and has come under repeated attacks in the past.
The United States has been militarily involved in Syria since 2014, but has for the most part avoided engaging directly in the country's civil war.
Tanf has been targeted before
Tanf has been the target of repeated attacks in recent months.
Last month, US-led allied forces and Syrian rebels repulsed a significant Daesh attack on the base.
On April 7, the US forces fired 59 Tomahawk missiles targeting Shayrat air base in Homs province after it was used in the regime chemical attack on the town of Khan Shaykhun.
Syria has been in a state of civil war since 2011 after the regime launched a crackdown on pro-democracy protesters.
More than 400,000 people have so far been killed in the conflict which has displaced half the population and triggered one of the world's worst refugee and humanitarian crises.