Iraq has become increasingly volatile as the civil war in neighbouring Syria strains fragile relations between Sunni and Shi'ite Muslims. Tensions are at their highest since U.S. troops pulled out of the country more than a year ago.
In the north of the capital Baghdad, gunmen attacked a police station and occupied it after killing five policemen, medics and police said.
In the western province of Anbar, a suicide bomber detonated his explosives vest in a group of government-backed Sunni "Sahwa" fighters who were collecting their salaries, killing six in a town east of the city of Falluja, police sources said.
"I was there to get my monthly salary as usual, and there was no security measures because the situation was normal ... a person drove his car among us, and suddenly the explosion happened," said Sahwa fighter Rasheed Muslih.
"When I opened my eyes, there was nothing but smoke and several of my colleagues were killed and the others wounded."
The "Sahwa" (awakening) councils, or the Sons of Iraq as they came to be known, have come under increasing attack from Sunni militants who despise them as allies of Shi'ite Prime Minister Nuri al-Maliki.
Sunnis have been protesting since December against Maliki, whom they accuse of marginalising their minority sect and monopolising power since U.S.-led troops toppled Saddam Hussein in 2003.
Government concessions had begun to take the edge off but when security forces raided a protest camp in the town of Hawija on April 23 and more than 40 people were killed, clashes spread to other Sunni areas.
In Baiji, 180 km (112 miles) north of Baghdad, police said a roadside bomb killed four policemen. A car bomb in a Shi'ite district in northeastern Baghdad killed at least three people, police and hospital sources said. Another car bomb north of the city of Ramadi killed two policemen and wounded another 10.
Iraq is home to a number of Sunni Islamist insurgent groups including a local al Qaeda affiliate that has launched frequent attacks to undermine the Shi'ite-led government and provoke wider confrontation.
Violence is still well below its height in 2006-07, but provisional figures from rights group Iraq Body Count put violent deaths in April at more than 400 - the highest monthly toll since 2009. About 1,500 people have been killed this year.
In the northern city of Mosul, unidentified gunmen shot a prominent tribal leader in a market and an electoral candidate was also assassinated in a separate incident, police said.
KURDS END BOYCOTT
Iraq's power-sharing government has been all but paralysed by disagreements between Sunnis, Shi'ites and ethnic Kurds, who run their own administration in the north of the country.
Kurdish ministers, who have boycotted government since March when parliament passed the 2013 budget without their consensus, said on Wednesday they would return to Baghdad, easing deadlock between the central government and the autonomous region.
A Kurdish delegation visited Baghdad earlier this week to address an ongoing row over land and oil, but there was no apparent breakthrough on that front.
Kurdistan, which has signed deals on its own terms with international oil firms, says it is owed more than 4 trillion Iraqi dinars. Baghdad rejects those contracts and has alloted the region a much smaller sum in the budget.