The US government says it will take “firm and appropriate measures” to protect a ceasefire in southern Syria if President Bashar Assad’s forces attack rebels there.
In a statement released on Friday, the State Department said it was concerned by reports that Assad’s forces were preparing for an operation near the Jordanian and Israeli borders.
The US, Russia, and Jordan agreed last year on a “de-escalation zone” in southwestern Syria to freeze the lines of conflict.
The State Department warned the Syrian government against “any actions that risk broadening the conflict”.
The US and Israel are growing increasingly concerned about Iranian military activities in Syria in support of Assad.
Israel says it does not want Iranian forces massing close to the nearby occupied Golan Heights.