SHURA, Iraq — Iraqi forces pushed into a town south of Mosul on Saturday after Islamic State fighters fled with civilians used as human shields, as state-sanctioned Shiite militias joined the offensive by opening up a new front to the west.

Iraqi troops approaching Mosul from the south advanced into Shura after a wave of U.S.-led airstrikes and artillery shelling against militant positions inside the town. Commanders said most of the IS fighters withdrew earlier this week with civilians, but that U.S. airstrikes had disrupted the forced march, allowing some civilians to escape.

“After all this shelling, I don’t think we will face much resistance,” Iraqi army Maj. Gen. Najim al-Jabouri said. “This is easy, because there are no civilians left,” he added. “The big challenge for us is always the civilians.”

Lt. Col. Hussein Nazim of the militarized Federal Police, said some civilians, mainly the elderly and infirm, might still be in the city, but that the use of heavy artillery and airstrikes was a standard tactic.

Iraqi forces launched a massive operation to retake militant-held Mosul last week. The offensive is expected to take weeks.