Iran said on Tuesday a new US-backed, 30,000-strong force inside Syria would "fan the flames of war", echoing the vehement response of Syria, Turkey and Russia to the plan.
Nearly 3 million people are believed to be in the Idlib region, their numbers swelled by fighters and civilians who fled Syrian army advances elsewhere in Syria.
Israeli jets and ground-to-ground missiles struck Syria early on Tuesday, Syria's army said, as Israeli Prime Minister Benjamin Netanyahu reiterated he would do what was needed to stop Hezbollah gaining "game-changing" Iranian weapons.
Russia said on Monday militants had used drones to attack its naval and air bases in Syria on Saturday, Russian news agencies reported.
Russia's defence ministry on Thursday said two servicemen were killed in a mortar attack by Islamist militants in Syria on New Year's Eve, but denied media reports seven military planes were destroyed.
Air strikes on one of the last villages still held by the Islamic State group in eastern Syria killed 12 civilians, including four children, a monitor said Monday.
Two mass graves containing dozens of bodies of civilians and Syrian troops killed by Islamic State jihadists have been found in the west of Raqqa province, state news agency SANA reported Friday.
Aid workers have begun evacuating emergency medical cases from Syria's besieged rebel bastion of Eastern Ghouta, the Red Cross said on Wednesday, after months of waiting during which the United Nations said at least 16 people died.