The warnings were included in 90,000 brightly-colored leaflets the U.S. dropped in the area over the weekend. Army Col. Ryan Dillon, spokesman for the anti-Islamic State coalition, said the forces' continued presence is threatening U.S. and allied troops. The coalition is prepared to defend them, he added.
The Pentagon has said the pro-Syrian-government forces are backed by Iran, a main ally of Syrian President Bashar Assad. But U.S. officials have said they're unsure if the forces are Syrian or Iranian, or members of Hezbollah or another militia.
Two versions of the bright red leaflets, written in Arabic, were dropped. One warns that any movement toward Tanf, a military camp near the Jordanian border where U.S. special operations forces are working with Syrian rebels, "will be seen as hostile intent and we will defend our forces." It directs the pro-Syrian troops to move back to another checkpoint, outlined in green on the paper.
The second leaflet says: "You are within an established decofliction zone, leave the area immediately." It also directs them to the checkpoint. The U.S. bombed pro-Syrian forces in the region earlier this month, after saying they refused to comply with similar warnings.
The Pentagon has said U.S. military commanders are relaying the warning to Russian officials in daily "deconfliction" phone calls designed to prevent accidents across Syria's crowded territory and in its increasingly crowded skies.