A military source on the Syrian government side said on Friday that a U.S. air strike against a Damascus-backed militia the day before hit "one of our military points", without elaborating, state media outlets reported.
The air strike late on Thursday killed several people and caused material damage, the source said, adding that this hampered efforts by the Syrian army and its allies to fight Islamic State.
The U.S. 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 U.S. and U.S.-backed rebel forces.
U.S. officials said the strike was purely a defensive measure.
A member of the U.S.-backed Syrian rebel forces told Reuters the convoy comprised Syrian and Iranian-backed militias and was headed toward the Tanf base when they clashed with some rebel forces.
Syria's ally Russia said on Friday that the strike had hit civilians and was unacceptable, Russian news agencies reported.
Russian Deputy Foreign Minister Gennady Gatilov, who the agencies said was speaking in Geneva, said the U.S. strike had violated Syria's sovereignty and would not help efforts to find a political solution to the conflict.
The U.S.-backed forces in that area are fighting against Islamic State militants. Separately, Syria's military supported by Russia and Iranian-backed militias are fighting against the jihadist group, and against more moderate rebel factions in the south.