ERBIL, Iraq -- 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 of Iraq and Syria (ISIS) militants, according to a top Iraqi commander.

A new analysis, meanwhile, has found that there was high risk that ISIS would deploy chemical weapons against Mosul civilians or Iraqi troops fighting to retake the city. According to IHS Markit, the extremist group has used chemical weapons at least 52 times in Iraq and Syria since its fighters swept across much of the two countries in 2014, including 19 times in the Mosul area alone.

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

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“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 still in Mosul - have combined to slow down progress in the campaign to liberate the city, which began Oct. 17.

Mosul, captured by ISIS in the summer of 2014, is Iraq’s second-largest city and the last major ISIS 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 ISIS 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 coalition airstrikes. Two Iraqi officers said the remaining two bridges will also likely be hit. 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.

According to the analysis by IHS Markit, a London-based intelligence analysis firm, Mosul served as a center for chemical weapons production for ISIS.

However, experts believe ISIS moved the materials and experts to Syria ahead of the Iraqi offensive to free the city. The analysis concluded that chlorine and mustard agents are the most likely chemicals to be used by ISIS in the Mosul battle.

In Budapest, visiting Iraqi Foreign Minister Ibrahim al-Jaafari told reporters that 1,700 militants have been killed and 120 captured since the battle for Mosul began. A third of Ninevah province, where Mosul is the capital, has been liberated so far, he said, adding that progress in the city has been slowed by the militants’ use of human shields.

He said that because of the country’s “extraordinary situation,” it would need to increase its crude oil output, which provides 90 percent of state revenues, and be exempt from OPEC quotas.