BAGHDAD: A suicide bomber attacked a government-backed militia in Iraq on Monday, killing at least 22 people in an apparent attempt by Sunni insurgents to provoke unrest against Shi'ite Prime Minister Nuri al-Maliki.

Dressed in civilian clothes, the bomber infiltrated a meeting of Sahwa tribal fighters and detonated his explosives as they picked up salaries in Taji, a town 20 km (12 miles) north of the capital Baghdad, police said.

It was the seventh suicide bombing in a month in Iraq, indicating insurgents are intent on stepping up violence a year after U.S. troops pulled out of the country, where Shi'ite, Sunni and ethnic Kurdish factions still struggle over how to share power.

"We got a call there had been a huge blast on the Sahwa headquarters in Taji. The Sahwa were there to collect their pay," said local police commissioner Furat Faleh. "When we rushed to the hall ... people were lying bleeding all around and cash was scattered in pools of blood."

The Sahwa or "Sons of Iraq" are former Sunni insurgents who rebelled against al Qaeda in the Sunni heartland province of Anbar at the height of the Iraq war and helped American troops to turn the tide of the conflict.

No group claimed responsibility for Monday's attack but al Qaeda's affiliate, Islamic State of Iraq, has often targeted Sahwa, pledging to take back ground lost to American and U.S. forces, and has urged Sunnis to rise up against Maliki.

The Shiite prime minister has been struggling to end mass protests by Sunni Muslims against what they see as the marginalisation of their sect since the fall of Saddam Hussein and the rise of Iraq's Shi'ite majority through the ballot box.

While violence has fallen from the height of the sectarian slaughter that killed tens of thousands in 2006-2007, insurgents have carried out at least one major attack a month since U.S. forces left in December 2011.

Sunni unrest and renewed violence are compounding fears the war in neighbouring Syria - where Sunni rebels are battling President Bashar al-Assad, an ally of Shi'ite Iran - could undermine Iraq's own delicate sectarian and ethnic balance.

Maliki's government has warned that al Qaeda and hardline Sunni Islamists may try to use the Syrian crisis as way to embolden Sunnis in Iraq and hijack protests.

A suicide bomber and gunmen killed at least 33 people at the police headquarters in the disputed northern oil city of Kirkuk on Sunday.

Last month, al Qaeda claimed responsibility for a suicide bombing that killed Sunni Muslim lawmaker Efan al-Essawi, a bitter foe of al Qaeda in the western city of Falluja.

Thousands of Sunni protesters have taken to the streets since late December, rallying mainly in Sunni-dominated Anbar province where they have blocked a highway to Jordan and Syria.

Many demonstrators want the government to amend laws that Sunnis believe authorities misuse to target them, but hardliners and Sunni Islamists have called for Maliki to step down and even for an autonomous Sunni fiefdom inside Iraq.

Al Qaeda's Iraq affiliate, which views Shi'ites as apostates, last month called on Sunni protesters to take up arms against the government.

"The escalation of attacks is an expected consequence of the awakening of al Qaeda sleeper cells encouraged by instability in mostly Sunni areas after the protests," said one Iraqi intelligence official.