BEIRUT — The United States warned it would take “firm and appropriate measures” to protect a cease-fire in southern Syria if President Bashar Assad’s forces move against rebels there.

The area in southwestern Syria, between the border city of Daraa and the Israeli-occupied Golan Heights, has emerged as a flashpoint in a wider standoff between regional archrivals Israel and Iran.

The U.S., Russia, and Jordan agreed last year to include Daraa in a “de-escalation zone” to freeze the lines of conflict. But government forces have recently dropped leaflets on rebel-held areas warning of an imminent offensive and urging fighters to lay down their arms, Syrian state media said Friday.

In a statement released Friday, the U.S. State Department said it was concerned by reports that Assad’s forces were preparing for an operation in southwestern Syria. It warned the government against “any actions that risk broadening the conflict.”

Assad has relied on Russia, Iran and the Lebanese militant group Hezbollah to battle an uprising against his family’s decades-long rule and roll back an Islamic State insurgency that grew out of the country’s seven-year civil war. Iran has sent military commanders to oversee battles and organize militias from across the Middle East to fight alongside Assad’s troops.

The U.S. and Israel view Iran’s extensive military presence in Syria as a threat to Israel and have threatened action. The Israeli military is believed to be behind dozens of airstrikes in recent years.