The army in a statement said the US-led anti-terror coalition attacked one of the military positions near the Tanf area on Thursday, leaving many personnel dead and causing some material losses, Xinhua news agency reported.
The official was "not sure" which forces were in the convoy but they "were clearly working with the Syrian" regime.
He was asked if the airstrike increases the US role in the Syrian war.
According to David Martin, CBS News national security correspondent, the USA attacked regime vehicles that had violated a deconfliction zone twice in three days. In Syria, Islamic State is fighting hard against several separate enemies, including Russian-backed Syrian government forces, trying to reinforce its presence as it is being pushed back in neighbouring parts of Iraq.
A deadly combination of human error, bad intelligence and an increasingly complicated battlefield led to the mistaken bombing of Syrian government troops by American and coalition aircraft in September.
Russia has denounced and called "absolutely unacceptable" a US-led coalition strike against a convoy of Syrian forces that violates Syria " s sovereignty, Russian Deputy Foreign Minister Gennady Gatilov said.
It also contains a key highway from Baghdad to Damascus that Iran has used to supply weapons to Syrian forces.
While officials stressed the defensive strike did not signal deepening USA involvement in Syria's civil war, it nonetheless raises questions about the ongoing feasibility for the coalition to maintain its singular focus on the so-called Islamic State group. Both forces have said they are trying to push Islamic State fighters out of the area.
USA -led troops helped repel a major IS attack on Tanf last month, while Russian warplanes bombed the base in June 2016.
The United States has led a coalition bombing IS in Syria since 2014, but in the past has avoided engaging with Syria or Iranian-backed forces battling rebel forces there.
Syrian government negotiator Bashar Ja'afari makes a statement after a meeting during the Intra Syria talks at the United Nations in Geneva, Switzerland May 19, 2017.
But the Syrian regime and its allies - including Iranian forces, Shiite militia, and Hezbollah - are also based some 40 miles outside Tanf, a rebel commander based there told BuzzFeed News last week, allowing them easy access to Tanf via a highway that cuts through the desert. That was followed by a "show of force" by aircraft overhead and warning shots, the officials said.