MOSUL, Iraq (AP) — Iraqi forces have secured less than a quarter of Mosul University as they face stiff resistance from Islamic State fighters in the battle for control of the city, according to senior Iraqi commanders overseeing the advance.

Convoys of Iraqi Humvees snaked through the sprawling university compound in eastern Mosul, pausing for artillery and airstrikes to clear snipers perched within classrooms, dormitories and behind the trees that line the campus streets.

Iraqi forces entered the university from the southeast Friday morning and by nightfall had secured a handful of buildings, Brig. Gen. Haider Fadhil and Lt. Gen. Abdul-Wahab al-Saadi said on a tour of the university Saturday.

Unlike in the surrounding neighborhoods, Iraqi officers said they believe the university grounds are largely empty of civilians and so they've been able to use air cover more liberally.

Thick clouds of black smoke rose over the sprawling complex Saturday morning. By afternoon, clashes had intensified with volleys of sniper and mortar fire targeting the advancing Iraqi forces.

Iraqi soldiers said their initial advance faced less resistance than they faced during the first weeks of the Mosul operation.

"We were targeted with only four car bombs where before (IS) would send 20 in one day," special forces Lt. Zain al-Abadeen said. "And they aren't armored like before, they're just using civilian cars."

The massive operation to retake Mosul, Iraq's second largest city, from IS was launched in October. Since then Iraqi forces have slowly clawed back about a third of the city.