Russia and Israel have reportedly reached a deal which will allow Syrian forces to take rebel territory in southern Syria.
The condition of the deal is for Iranian troops to not participate in the offensive, and in return, Israel will not stand in the way of any Syrian regime offensive along the Israeli and Jordanian border.
Southern Syria is of concern to the United States, which last year brokered a "de-escalation" deal with Jordan and Russia that has largely contained the war near the frontier with Israel.
The US warned it would take "firm and appropriate measures" in response to any violations of that ceasefire.