"On Thursday at 16:30, the so-called worldwide coalition attacked one of the Syrian Arab Army's positions on the Al-Tanf road in the Syrian Badia region, producing a number of martyrs and causing material damage", a military source told Syrian state media.
One of the defense officials - who, like the others, spoke on condition of anonymity to reveal details that the Pentagon had yet to make public - said that the United States acted to stop an advance of pro-regime forces toward Tanf.
A member of the US-backed Syrian rebel forces told Reuters the convoy comprised Syrian and Iranian-backed jihadists and was headed toward the Al-Tanf base when they clashed with some rebel forces.
The United States is not deepening its role in Syria's civil war but will act defensively when needed, US Defence Secretary Jim Mattis said on Thursday.
Russian Deputy Foreign Minister Gennady Gatilov also said the strikes had targeted "the Syrian armed forces".
Such deliberate airstrikes against pro-regime forces are extremely rare, with the last such strike being carried out in April in retaliation for the use of chemical weapons.
The official was "not sure" which forces were in the convoy but they "were clearly working with the Syrian" regime.
The Syrian Observatory for Human Rights, a UK-based monitoring group, reported that eight people were killed.
Pro-Assad outlets reported that the USA -led coalition hit the Iranian-backed Imam Ali brigade.
A US airstrike has hit pro-Syrian government forces in southern Syria, US officials say.
The statement said coalition forces have been operating in the area "for many months training and advising vetted partner forces" in the battle against IS. This is totally unacceptable; it is a violation of Syrian sovereignty.
"We notified the coalition that we were being attacked by the Syrian army and Iranians at this point and the coalition came and destroyed the advancing convoy", Saloum said.
In April the USA launched 59 cruise missiles at a Syrian military airfield in retaliation for a chemical attack that killed scores of civilians.
The coalition aircraft also used warning shots in an effort to dissuade the forces from establishing the fighting positions near the base. According to reports, the strikes were originally intended as a show of force to deter a 20-vehicle pro-regime convoy from heading toward the base.
After the ground forces continued their activity, the US carried out the airstrike, the officials said.
Later, Secretary of Defence Jim Mattis stressed that USA is not getting more involved in Syria's civil war.