Trying to stem the infiltration of militia fighters, American forces have begun to build a massive concrete wall that will partition Sadr City, the densely populated Shiite neighborhood in the Iraqi capital.

American soldiers scanned roofs in Sadr City on Thursday after shots were fired near their base.

The construction, which began Tuesday night, is intended to turn the southern quarter of Sadr City near the international Green Zone into a protected enclave, secured by Iraqi and American forces, where the Iraqi government can undertake reconstruction efforts.

“You can’t really repair anything that is broken until you establish security,” said Lt. Col. Dan Barnett, commander of the First Squadron, Second Stryker Cavalry Regiment. “A wall that isolates those who would continue to attack the Iraqi Army and coalition forces can create security conditions that they can go in and rebuild.”