BEIRUT—The United States warned it would take “firm and appropriate measures” to protect a ceasefire 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 a Daesh 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 against Hezbollah, Iran and Syrian military positions.

Earlier this month, Israel bombed Iranian military positions in Syria in what it said was retaliation for an Iranian rocket attack on the occupied Golan Heights. Israel called it its most serious operation in Syria since the 1973 Mideast war.

The government began moving reinforcements to Daraa province this week after expelling the last rebels and Daesh militants from around Damascus, according to the Britain-based Syrian Observatory for Human Rights, which closely monitors the war.

Leaflets dropped on northern Daraa, which is divided between rebel and government-controlled areas, warned: “The men of the Syrian army are coming.”

The Syrian uprising began in Daraa in 2011. The ceasefire has slowly disintegrated as government warplanes have carried out airstrikes against rebel-held areas.

Meanwhile in Idlib, a car bomb in one of the northern city’s main streets killed at least four people and wounded about 30 others on Saturday, according to the Syrian Civil Defence search-and-rescue group.

Idlib, the capital of a province by the same name, has suffered deterioration to its security in recent months as rebel and jihadist factions battle with the al-Qaida-linked Levant Liberation Committee for dominance. The province is one of the opposition’s last remaining footholds in the country. The other is Daraa.

It was not clear who was behind Saturday’s bomb blast. According to the Observatory, at least 119 people have been killed over the past month in the factional infighting in Idlib. Thirty-one of them have been civilians.

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