The Iraqi military’s operation to root out Islamic State militants from Tikrit this week has empowered a new generation of militias, which emerged from relative obscurity to take a lead role in the battle.

Now, U.S. and Iraqi officials are hoping these irregular Shiite troops, which they say aren’t beholden to Iran and are more careful to avoid stoking sectarian tensions with the Sunni community, will help defeat the Sunni militants across Iraq.

One of the challenges to this plan can be found in the hills surrounding Tikrit, where thousands of Iran-backed militia members who helped in the initial stages of the monthlong fight for the city are amassed. These troops are resentful after being sidelined as a condition for U.S. airstrikes that helped liberate the city—the largest to have been reclaimed from the extremist’s forces so far—and their responses are unpredictable.

U.S. officials also warned that the Tikrit fight is unlikely to be finished. Iraqi forces were still clearing the city on Wednesday and attacks from remaining Islamic State fighters could hamper the city’s submission.

Still, U.S. officials were heartened by the apparent ease with which Iraqi forces swept into the city a relatively short time after American and allied aircraft joined the Tikrit fight last week.