Syrian government forces and their Russian allies pounded the rebel-held northwest with air strikes on Saturday, the fifth day of a campaign that has killed dozens of people and forced thousands to flee, sources in the area and a war monitor said.
Artillery that may have been fired by Syrian government forces struck near a Turkish military post in the northwest, a Turkish security official said, signalling the risks of escalation in a part of Syria where Turkish troops have deployed under a deal with Russia.
The upsurge in violence in Idlib and nearby areas has strained a Russian-Turkish deal that has staved off a government offensive since September. The area is part of the last major foothold of the rebellion against President Bashar al-Assad.
Assad, who has defeated many of his enemies aided by Russian and Iranian firepower, has vowed to recover every inch of Syria.
But the presence of Turkish forces in the northwest and Russian understandings with Ankara have complicated any offensive into the region, home to some 3 million people.
Turkey, already hosting 3.6 million Syrian refugees and hoping to avoid another influx, has established a dozen military positions in Idlib and nearby areas under agreements with Russia.
The Turkish official said the army had not yet retaliated to the attack and there were no reports of casualties.
Russian President Vladimir Putin said last week he did not rule out Syrian forces, backed by Russian air power, launching a full-scale assault on militants in Idlib, but such an operation was impractical for now.
Syrian state media say government forces are attacking jihadists in the northwest. State news agency SANA said the army had destroyed jihadist positions in southern Idlib and nearby Hama province on Saturday, in response to what it called repeated violations of a de-escalation agreement.
But the U.N. regional humanitarian coordinator has said schools, health facilities and residential areas have been hit and the government forces are employing the worst barrel bombing in at least 15 months.
Barrel bombs are containers packed with explosives dropped from helicopters.
A rebel spokesman said government attempts to advance into the Qalaat al-Madiq area had been repelled. Rebels were shelling government positions, added Naji Mustafa of the Turkey-backed National Liberation Front (NLF) rebel grouping.
After an overnight lull, the bombardment escalated again on Saturday, said Ahmad al-Dbis, safety and security manager for the U.S.-based Union of Medical Care and Relief Organizations (UOSSM), which supports medical facilities in the area.
"Now the bombing has returned and is much heavier and has spread very widely in Jabal al-Zawiya and rural northern Hama. The planes are not stopping at all and the bombing is continuing in a very big way like yesterday and worse," he added.
The Syrian Civil Defense, a rescue service operating in rebel-held areas, said it had recorded more than 30 deaths in the last few days. Dbis said the number of dead was at least 50, while the UK-based Syrian Observatory for Human Rights, which reports on the war, said at least 67 people had been killed.
Hundreds of vehicles have been arriving every day in the town of Atmeh at the Turkish border, ferrying people away from the targeted areas, an Atmeh resident contacted by Reuters said.
Mustafa al-Haj Yousef, the Civil Defense director for Idlib, said more than 130,000 people had fled towards more secure areas, adding: "Civil Defense centres have been targeted directly."
UOSSM says four medical facilities have been bombed.
Russia's deal with Turkey demanded the creation of a demilitarised zone free of all heavy weapons and jihadists. But Moscow says the agreement has not been implemented.
The most powerful faction in the northwest is Tahrir al-Sham, a jihadist movement that emerged from the Nusra Front, formerly al Qaeda's official Syrian affiliate. Its influence has grown as it has snuffed out rival groups. But other factions operating under the NLF umbrella still have a presence.