U.S. forces for the first time bombed a convoy of Syrian government and Iranian-backed militia forces after Russia was unable to keep them from entering an area where U.S. allies have been preparing to battle the Islamic State, the Pentagon said on May 18.
The Syrian-allied forces were threatening U.S. troops and allied rebel forces operating near the town of Al-Tanf on Syria's border with Jordan, and were hit with air strikes after Russian intervention and warning shots failed to deter them, the Pentagon said.
The attack left eight of the fighters dead, the Syrian Observatory for Human Rights said.
Asked if the move signalled a broadening of U.S. involvement in the six-year Syrian civil war, U.S. Defense Secretary Jim Mattis said, "We are not increasing our role in the Syrian civil war, but we will defend our troops."
Officials said the strikes were also intended as a signal to Syrian President Bashar al-Assad to keep his forces out of the region where the borders of Jordan, Syria, and Iraq meet, which was designated a deconfliction zone under an agreement between the United States and Russia.
It also contains a key highway from Baghdad to Damascus that Iran has used to supply weapons to Syrian forces.
The Syrian Observatory said most of the eight people killed in the strikes appeared to be "non-Syrian." It said four vehicles also were destroyed.
The United States has led a coalition bombing IS in Syria since 2014, but in the past has avoided engaging with Syria or Iranian forces battling rebel forces there.
The Pentagon said it has been operating in the Al-Tanf area for months training partner forces to battle IS.
Elsewhere in Syria on May 18, IS attacked several government-held villages in central Syria, capturing at least one of them.
The attack left 52 people dead, including more than two dozen women and children, some of whom were beheaded, as well as Syrian troops, the monitoring group said.
The Observatory said it was the most violent IS attack so far this year.