A suicide bomber on Monday blew himself up near a group of anti-Qaeda militiamen as they were receiving their salaries in the town of Taji, killing four of them, an interior ministry official said.
The attack in Taji, around 25 kilometres (16 miles) north of Baghdad, came a day after separate attacks killed 30 people in north Iraq.
The Sunni militiamen, known as Sahwa (Awakening), were receiving their salaries when a bomber "wearing an explosives belt detonated his charge, killing four militiamen and wounding 20," the official said.
A medic working in Baghdad's Kadhimiya hospital confirmed the toll.
Sahwa militiamen, mostly Sunni Muslims who helped turn the tide of violence which engulfed Iraq in the wake of the US invasion, took up arms against Al-Qaeda in Iraq in 2006. They have often been targeted by the extremist group.
The Sahwa fighters recently saw their monthly salary, paid by the government, rise to 500,000 dinars (around 415 dollars) from 300,000 dinars.
Shiite Prime Minister Nuri al-Maliki's government is hoping this gesture will appease Sunni protesters who have been demonstrating against his rule for more than a month.
Violence was also reported in other parts of Iraq on Monday and during the night.
In the western Baghdad neighbourhood of Jihad, a police lieutenant was killed and three of his colleagues wounded in a blast caused by an improvised explosive device, the ministry official and medics said.
Four people were also shot dead overnight in Kirkuk, hours after a coordinated assault on a police headquarters in the north Iraq city left 30 people dead, officials said.
The security and medical officials said that at around midnight a gunman opened fire on four men sitting in a caravan adjacent to a neighbourhood electricity generator, killing all of them.
Districts nationwide are serviced by private generators which fill the large power gap, as most Iraqis get fewer than 10 hours of electricity per day.
Typically, the generator operator and some of his friends stay in a caravan next to the machine to ensure it operates constantly.
The violence comes as Iraq grapples with a political crisis pitting premier Maliki against his government partners amid weeks of protests calling for him to resign.
No one has claimed responsibility for the attacks but local security officials blame Al-Qaeda's front group in Iraq, which often targets security forces and officials in a bid to destabilise the country and push it back towards the sectarian bloodshed of 2005 to 2008.
Kirkuk, an ethnically mixed city 240 kilometres (150 miles) north of Baghdad, lies at the heart of a swathe of disputed territory claimed by both the central government and Iraq's autonomous northern Kurdish region.
The unresolved row is persistently cited by diplomats and officials as the biggest threat to Iraq's long-term stability.
The violence was the latest in a spike in unrest that saw 246 people killed last month, the most since September, according to an AFP tally.