BEIRUT - 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.