Iraq’s military said it was closing in Friday on a former government compound in Ramadi that remains the only area of the city still under Islamic State control.

Iraqi forces were less than a mile from the compound where about 100 militants are holding out, the government said. Though Iraqi troops greatly outnumber the Islamic State fighters, they have struggled to advance in a landscape dotted with land mines and improvised explosive devices.

“Our preparations to attack the government compound are still on and we are approaching step by step,” Brig. Gen. Ahmed al-Bilawi, commander of the Anbar province emergency police division, said Friday.

Iraqi forces backed by U.S-led airstrikes started a new push earlier this week to retake Ramadi, the provincial capital of Anbar about 60 miles west of the capital Baghdad.

Though Kurdish fighters and Iran-backed Shiite Muslim militias have led most victories against Islamic State in the country, most of the fighting in Ramadi has been done by Iraqi forces recently retrained and re-equipped by the U.S.

Iraqi and American military leaders see the battle for Ramadi as a testing ground for a future assault on Mosul, Iraq’s second-largest city and the extremist group’s de facto capital in the country. But given the army’s difficulties in Ramadi, any plans to retake the much-larger Mosul are expected to prove more challenging.

Islamic State fighters have had more than a year and a half to establish themselves in Mosul since storming the city in the summer of last year. They have built defenses including land mines and possibly tunnels, tactics used in other cities recaptured from the group, while ruling over hundreds of thousands of civilians who could be used as human shields.

Civilians who remain trapped in Ramadi have posed a formidable challenge for both the Iraqi troops and their U.S. air support. At least 120 families are trapped between Islamic State fighters and invading Iraqi soldiers in the eastern suburbs of Sejariya and Sofiya, said Mohamed al-Khozaee, a spokesman for the Iraqi Red Crescent Society.

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