“The militias have even bigger role now that they are said to be fighting ISIS” said Alla Maki, a Sunni lawmaker. “Who will control them? We have no real Iraqi Army.”

Under former Prime Minister Nuri Kamal al-Maliki, Asaib Ahl al-Haq was “encouraged to do dirty jobs like killing Sunnis, and they were allowed to operate freely,” Mr. Maki said. “Now the international community are all being inspired by the removal of Maliki personally, but the policy is still going on.”

The Asaib Ahl al-Haq fighters and the group’s official spokesman insisted that their vigilante attacks protect all Iraqis, Sunnis as well as Shiites. “We have been able to track the sleeper cells of ISIS and secure almost all of Baghdad — about 80 percent,” said Naeem al-Aboudi, a spokesman for Asaib Ahl al-Haq, in a gleaming, leather-paneled conference room at its heavily fortified headquarters in an elite neighborhood of the capital.

In the current fight, he added, “the most dangerous areas in Iraq were assigned to Asaib Ahl al-Haq to lead the battle, because of the capability and professionalism of our fighters.”