Syrian regime forces shelled the south of Idlib on Sunday, according to rescuers and residents in the opposition 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. An opposition 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. Syrias northwest corner, including the Idlib region, is the last major chunk of territory still in opposition 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. David Miliband, who heads the aid group International Rescue Committee, told a UN panel on northwest Syria Friday that the breakdown of the recent ceasefire "makes us fear that the worst may be yet to come." He called for an immediate end to the violence against civilians and an accountability mechanism to investigate attacks against civilians. 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 fighters there, and preventing a fresh refugee wave into Turkey. In early August, another ceasefire had collapsed within three days, after which the army pressed its offensive and advanced along a main highway. Bouthaina Shaaban, a senior adviser to regime head Bashar Assad, 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 opposition territory in Idlib towards army lines. It accused militants of preventing people from leaving. State news agency SANA said militants shot at motorists, denying them an exit. There was no independent confirmation. The Civil Defense, rescue workers operating in opposition territory, said artillery shelling on Idlib villages killed seven people since Friday. Ankara, which backs some opposition factions in northwest Syria, brokered a "de-escalation" deal with Russia in 2017 that sought to curb fighting in Idlib. It does not cover militants. Under its deals with Russia and Iran, Turkeys military also has a dozen posts in the Idlib region, which had come in the firing line during the latest Syrian army advance.