BAGHDAD — A suicide bomber attacked fighters opposed to the “Islamic State” as they gathered Wednesday to receive salaries south of Baghdad, killing at least 33 people and wounding 55 others, Iraqi police and medical officials said.

“The attacker was wearing an Iraqi army uniform, and an explosive vest packed with ball bearings,” a police officer said.

The attack was on a group known as Sahwa, or “Awakening” in Arabic, which dates back to the height of the U.S.-led war in Iraq, when Sunni tribesmen joined forces with the Americans to battle insurgents including IS’s predecessor organization, al-Qaeda in Mesopotamia.

Recently, the Iraqi government began distributing arms and ammunition to tribesmen and other groups, including Sahwa, as Prime Minister Haidar al-Abadi seeks to establish an anti-IS national guard made up of local fighters.

The dead were mostly anti-IS fighters, but included at least three soldiers, Iraqi officials said.

There was no immediate claim of responsibility for the attack, but suicide bombings are a tactic almost exclusively employed by extremist groups in Iraq, including ISIS.

Over the past weeks, the Iraqi army, backed by Kurdish forces, volunteering fighters and tribesmen have clawed back some ground from IS, inflicting a string of defeats on the jihadists.