Coalition fighter jets on Thursday struck a convoy of militiamen advancing inside a protected "deconfliction zone" north-west of the southern town of At Tanf, the military alliance said in a statement.
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The US, which is leading an air campaign in Syria targeting groups such as the Islamic State of Iraq and the Levant (ISIL, also known as ISIS), said the convoy's advance had posed a threat to US and US-backed Syrian rebel forces in the area.
"This brazen attack by the so-called international coalition exposes the falseness of its claims to be fighting terrorism," a Syrian military source told state media on Friday, confirming that the bombing had killed "a number of people" and caused material damage.
Russia, which launched its own air campaign in September 2015 in support of Syria's President Bashar al-Assad, called the strike "a breach of Syrian sovereignty".
"Such actions that were carried out against the Syrian armed forces ... [are] completely unacceptable," Gennady Gatilov, Russia's deputy foreign minister, was quoted as saying by state-run RIA Novosti on Friday.
A member of the US-backed Syrian rebel forces told the Reuters news agency that the convoy comprised Syrian and Iranian-backed militias and was headed towards the Tanf base,  where US special forces operate and train Free Syrian Army rebels.
Rami Abdel Rahman, head of the Syrian Observatory for Human Rights (SOHR), said at least eight people had been killed in the attack.
"Most of the killed belong to militias loyal to the Syrian regime and are not Syrians," he told the DPA news agency.
SOHR, a UK-based monitor tracking developments in Syria's long-running conflict via a network of contacts on the ground, also said that four military vehicles carrying pro-government forces and their allies were destroyed in the strike.