Syrian troops shelled the south of Idlib on Sunday, according to rescuers and residents in the rebel stronghold where a ceasefire had halted a fierce army offensive two weeks ago.
Artillery fire battered Maarat al-Numan town and nearby villages in the south Idlib countryside over the past two days, after warplanes struck there on Thursday.
A rebel official said fighters were on high alert and had reinforced the frontlines. "We are responding directly by targeting the positions from where the shells are fired," said Naji Mustafa of the National Liberation Front insurgent force.
Syria's northwest corner, including the Idlib region, is the last major chunk of territory still in rebel hands after more than eight years of war.
The ceasefire Damascus declared on Aug. 31 brought a lull in air strikes, after a five-month offensive which the United Nations says killed hundreds of people.
Syrian President Bashar al-Assad's government and his key ally Moscow have said they were responding to attacks by militants formerly linked to al-Qaeda.
Turkish President Tayyib Erdogan will host the presidents of Russia and Iran on Monday. Talks will focus on securing an Idlib truce, asserting control over jihadists there, and preventing a fresh refugee wave into Turkey.
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In early August, another ceasefire had collapsed within three days, after which the army pressed its offensive and advanced along a main highway.
A senior Assad adviser, Bouthaina Shaaban, described the ceasefire this month as a temporary means to serve the strategy of "liberating every inch" of the country.
Syrian state TV said on Sunday that troops had opened a corridor for residents who want to cross out of insurgent territory in Idlib towards army lines. It accused miiltants of preventing people from leaving.
The Civil Defence, rescue workers operating in opposition territory, said artillery shelling on Idlib villages killed seven people since Friday.
Ankara, which backs some rebel factions in northwest Syria, brokered a "de-escalation" deal with Russia in 2017 that sought to curb fighting in Idlib. It does not cover jihadist militants.
Under its deals with Russia and Iran, Turkey's military also has a dozen posts in the Idlib region, which had come in the firing line during the latest Syrian army advance.