A US military strike in Syria on Thursday was "government terrorism" and caused a massacre, Syrian government negotiator Bashar al-Ja'afari said on Friday.
He said he had raised the issue in talks with U.N. mediator Staffan de Mistura.
"We discussed the massacre that the U.S. aggressor committed yesterday in our country. This subject was widely discussed," Ja'afari told reporters.
"The important thing is that our political ambition is higher because we want to focus on fighting terrorism represented by armed groups and the state and government terrorism happening against our country. This includes the American aggression, French aggression and British aggression."
A military source on the Syrian government side says a US air strike against a Damascus-backed militia hit "one of our military points" and killed several people.
The air strike late on Thursday also caused material damage, the source said, adding that this hampered efforts by the Syrian army and its allies to fight Islamic State.
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.
A member of the US-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 the strike had hit civilians and was unacceptable, Russian news agencies reported.
Russian Deputy Foreign Minister Gennady Gatilov Gatilov, who the agencies said was speaking in Geneva, said the US strike had violated Syria's sovereignty and would not help efforts to find a political solution to the conflict.
The US-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.