U.S. warplanes have attacked a convoy of Syrian regime-backed fighters who violated an established deconfliction zone in southern Syria, a Pentagon official told the Washington Examiner on Thursday.

The strike took place near the al-Tanf border crossing on Syria's southern border with Jordan and Iraq, where U.S. special forces have been training counter-Islamic State troops for many months.

A senior military official said the convoy of vehicles appear to be probing the zone, which is recognized by the Russians as an area in which U.S. troops are operating.

"They were trying to see how close they could get," the official said.

The U.S. went through a series of steps to try to get the convoy to turn back, beginning with a call to the Russians on a special deconfliction channel, followed by a "show of force" with a flyover of U.S. warplanes.

In what the official called a gradual escalation of forces, the official said warning shots were fired, and when that did not deter the convoy, some of the vehicles were bombed.

The U.S. said the pro-regime forces violated a deconfliction zone around the training base at al-Tanf.

The U.S. had no report of casualties on the ground, but the official said presumably some of the Syrian fighters were killed.