A suicide bomber struck a group of anti-al-Qaeda fighters north of Baghdad on Monday, killing at least 22 people and wounding 44 in an attempt to shake confidence in Iraqi security forces, officials said.
The blast is the latest in a string of particularly lethal attacks against security forces and civilians. More than 200 Iraqis have been killed since the beginning of the year.
In Monday's bombing, the attacker mingled with men gathering to collect their salaries outside one of the anti-al-Qaeda militia's headquarters in the town of Taji, about 20 kilometres north of the Iraqi capital.
Three Iraqi soldiers and 19 members of the Sunni militia known as Sahwa, or Awakening Councils, were killed, police officials said.
Three medical officials confirmed the casualties. All officials spoke on condition of anonymity because they are not authorized to release information to reporters.
The Sahwa was formed from Sunni fighters who switched sides and joined U.S. and Iraqi government forces to fight al-Qaeda at the height of the insurgency. Since then, they have been among the favourite targets for insurgents, who see them as traitors.
Growing discontent among minority Sunnis
The spike in especially bloody attacks comes at a time of growing discontent among Iraq's minority Sunnis, who complain of discrimination by the Shia-led government.
In recent weeks, Sunnis have staged frequent anti-government demonstrations drawing tens of thousands of people. At the same time, protest organizers have distanced themselves from calls by an al-Qaeda front in Iraq to take up arms against the government.
The blast in Taji came a day after several suicide attackers on foot and in two explosives-laden cars hit a provincial police headquarters in Kirkuk, also north of Baghdad.
The deputy police chief in Kirkuk, Maj. Gen. Torhan Abdul-Rahman Youssef, said Monday that 16 people were killed in that attack, dismissing initial reports of 30 dead.
About 90 people were wounded in the Kirkuk explosion.
No group has claimed responsibility for the latest attacks, but suicide bombings are a hallmark of al-Qaeda in Iraq.
Violence has ebbed across Iraq since the peak of the fighting in the last decade, but deadly bombings and shootings still occur almost daily.