The Syrian Observatory for Human Rights reported "around 3,000 people" returned to the northern Hama and the southern Idlib countryside from other areas under regime control.
Syrian regime 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 opposition force.
The fighting is taking place as some displaced Syrians are returning to the areas they previously fled.
"Thousands of citizens return to their villages and towns of the northern Hama countryside and the southern Idlib countryside," state news agency SANA said.
The Syrian Observatory for Human Rights, a Britain-based monitor, reported "around 3,000 people" going home from other areas under regime control.
Since August 31, a ceasefire announced by regime backer Russia has largely held in northwestern Syria, though the Observatory has reported sporadic bombardment.
State media said the returns came amid "government efforts to return the displaced to their towns and villages".
The Idlib region of around three million people, many of them displaced by fighting in other areas, is one of the last holdouts of opposition to forces backing regime leader Bashar al Assad.
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 August 31 ushered in a lull in air strikes, after a five-month offensive which the United Nations says killed hundreds of people.
Assad's regime and it's key ally Moscow have said they were responding to attacks by militants formerly linked to al Qaeda.
Turkish President Recep Tayyip Erdogan will host the presidents of Russia and Iran on Monday.
Talks will focus on securing an Idlib truce, asserting control over militants there, and preventing a fresh refugee wave into Turkey.