MOSUL, Iraq (AP) — Iraqi forces have retaken the eastern edges of three of Mosul's five bridges, according to a senior Iraqi officer overseeing the operation to retake Iraq's second-largest city from the Islamic State group.

The retaken bridges are in the city's southeast and come as Iraqi forces also advance in the northeast, where they have begun battling IS fighters on the grounds of Mosul university. Warplanes from the U.S.-led coalition bombed the city's bridges late last year in an effort to isolate IS fighters in the city's east by disrupting resupply routes.

At Mosul University, senior commanders said that Iraqi forces have secured just under a quarter of the campus amid stiff resistance. 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.

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. Convoys of Iraqi Humvees snaked through the complex, pausing for artillery and airstrikes to clear snipers perched within classrooms, dormitories and behind the trees that line the campus streets.

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.

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 from IS was launched in October. Since then Iraqi forces have slowly clawed back about a third of the city.

Associated Press writer Qassim Abdul-Zahra in Baghdad contributed to this report.