The U.S. military carried out an air strike on May 18 against militia supported by the Syrian government that posed a threat to U.S.-backed fighters in the country's south near the Jordanian border, U.S. officials say.
One of the U.S. officials said the strike near the town of At Tanf destroyed at least one tank and a bulldozer.
Another official said the strike came after U.S. aircraft fired warning shots meant to discourage the fighters from advancing.
A member of the U.S.-backed Syrian rebel forces said the strikes hit a convoy of Syrian- and Iranian-backed militias that were heading toward an At Tanf base used by U.S. and U.S.-supported forces, including elite U.S. and British commandos.
The warplanes struck after rebel fighters clashed with Syrian and Iranian militias who had advanced to less than 30 kilometers from the base, said Muzahem al Saloum, from the Maghawir al Thwra group.
"We notified the coalition that we were being attacked by the Syrian Army and Iranians in this point, and the coalition came and destroyed the advancing convoy," Saloum said.
The Pentagon did not immediately respond to requests for comment.
The strikes would be the first against fighters aligned with Syria's government since the United States launched cruise missiles on a Syrian air base last month.
The April strikes were ordered in retaliation for a chemical weapons attack that Washington blamed on Damascus.