Islamic State militants are grabbing human shields as they fall back toward the center of Mosul in the face of advancing Iraqi forces, U.S. officials and aid agencies said Thursday.

The militants have established “disruption zones” outside Mosul, the second-largest city in Iraq, where the Islamic State is attempting to inflict casualties on Iraqi forces as it withdraws toward the city to make a final stand. Some of the militants are taking civilians with them as they retreat to the city.

Militants also prevent civilians from fleeing areas where there is fighting between the Islamic State and Iraqi forces, Amnesty International said in a statement Thursday. As many as 1 million civilians remain in Mosul, according to the Pentagon.

The Islamic State, also known as ISIL or ISIS, created networks of berms, tunnels and roadside bombs in these disruption zones outside the city, Air Force Col. John Dorrian, a coalition military spokesman, said Thursday.

U.S.-led coalition airstrikes have destroyed 40 tunnels and more than 160 militant fighting positions since the Mosul operation began 10 days ago, Dorrian said.

Fighters can engage the Iraqi security forces in the zones and then retreat toward the city center. By grabbing civilians the militants hope to avoid coalition airstrikes.

“We expect a very hard defense in-depth, in the center of the city probably,” Lt. Gen. Stephen Townsend, the top coalition commander in Iraq, told Pentagon reporters this week.

The militants have frequently used human shields to defend against coalition airstrikes, since pilots will not launch attacks unless they are certain civilians are not around.

Advance elements of Iraqi and Kurdish forces are about 6 miles from the edge of Mosul.