IRBIL, Iraq (AP) — Iraqi troops moved on Tuesday to retake another neighborhood in the eastern sector of the northern city of Mosul but were facing stiff resistance from Islamic State militants, according to a top Iraqi commander.

Brig. Gen. Haider Fadel of the special forces told The Associated Press that IS fighters were targeting his forces with rockets and mortars as they slowly advanced in the densely populated Zohour neighborhood.

"We are cautiously advancing. There are too many civilians still living there," he said.

Iraqi troops began their siege of Zohour on Sunday as they fortified their positions in neighborhoods they had already retaken in eastern Mosul. Suicide bombings, sniper fire and concerns over the safety of civilians — there are 1 million civilians still in Mosul — have combined to slow down progress in the campaign to liberate the city, which began Oct. 17.

Mosul was captured by IS in the summer of 2014. It is Iraq's second-largest city and the last major IS urban bastion in the country. Most gains in the campaign so far have been made by the special forces operating east of the Tigris River. Other forces, including the Kurdish Peshmerga troops and volunteer Sunni militiamen, are advancing on the city from different directions, and the U.S.-led coalition is providing airstrikes and other support.

An airstrike by the U.S.-led coalition on Monday destroyed a major bridge over the Tigris in the southern part of the city, a move that appears designed to limit the IS capacity to reinforce or resupply fighters on the east bank of the Tigris where most of the fighting is taking place.

It was the third of the city's five bridges on the Tigris to be targeted by the coalition — the first two were destroyed in airstrikes shortly before and after the start of the Mosul campaign — a sequence that two Iraqi officers said was likely to soon extend to the remaining two bridges to completely separate the city's eastern sector from the western bank of the Tigris.

The officers spoke to the AP on condition of anonymity because they were not authorized to speak to the media.

The Iraqi military is known to have received U.S.-made pontoon bridges which Iraqi troops would use as a substitute for the destroyed bridges.