REUTERS

The militia, who numbered in the dozens and drove a tank and a small number of construction vehicles, ignored warning shots from U.S. aircraft and, according to a U.S.-led coalition statement, even "apparent Russian attempts to dissuade" their advance, as reported by Reuters.

One of the U.S. officials, who spoke on condition of anonymity, speculated that the group might have been trying to establish a position near the garrison in Syria used by U.S. and U.S.-backed forces around the town of At Tanf.

"They were potentially probing to see how close they could get to At Tanf," the official said.

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A member of the U.S.-backed Syrian rebel forces told Reuters the convoy comprised Syrian and Iranian-backed militias and was headed toward the Tanf base when they clashed with some rebel forces.

Read alsoRussia sends more weapons to Syria ahead of Lavrov visit to Washington – Fox NewsThe militia were struck after they had advanced to about 17 miles (27 km) from the base.

"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," said Muzahem al Saloum of the Maghawir al Thwra group.

Since they appeared defensive in nature, Thursday's strikes did not suggest a shift in the U.S. military's focus in Syria, which has been on battling Islamic State militants.

U.S. Defense Secretary Jim Mattis said America's role in Syria's conflict was unchanged.

"No. We are not increasing our role in the Syrian civil war. But we will defend our troops," Mattis said when asked about the strikes.