Special forces have clashed with hundreds of Sunni militants in two Iraqi cities.
The clashes with the fighters from the al-Qaeda linked Islamic State of Iraq and the Levant (ISIS) were in Fallujah and Ramadi, where some 250 fighters were equipped with armed vehicles.
Both cities are in the province of Anbar, where Sunni militant activity has been on the rise.
At least 23 people were killed in militant attacks elsewhere in Iraq.
Sunni Arabs in Iraq claim they have been targeted and politically marginalised by the Shia-led government of Prime Minister Nouri al-Maliki, a charge the prime minister denies.
The fighting in Ramadi was reported to have eased by Thursday evening, but it was not clear whether authorities had totally re-established control in either city.
A senior police officer in Anbar province had earlier been quoted as saying ISIS fighters had seized at least 10 police stations and had freed a number of prisoners.
ISIS fighters posted videos of themselves burning government vehicles, setting up checkpoints and issuing challenges to the authority of Mr Maliki.
Sunni tribal fighters also took to the streets, with reports that some were fighting on the government side.
Also on Thursday, authorities arrested a Shia militia leader in what may be seen as an attempt to appease Sunni sentiment.
Wathiq al-Batat is the commander of the Mukhtar army, a recently formed Shia militia which was implicated in a rocket attack last week.
Attacks stepped up
On Monday tensions in Anbar were heightened when security forces broke up a Sunni anti-government protest camp in Ramadi.
As well as the breaking up of the protest camp, the arrest of a Sunni MP at the weekend raised tensions.
Ten people died when the government moved against the camp in Ramadi on Monday.
In April, a raid by security forces on a similar protest camp in the northern town of Hawija left 44 civilians and one policeman dead.
The move by the authorities came after Mr Maliki said the protest camp had "turned into a headquarters for the leadership of al-Qaeda".
Extremist Sunni militants linked to al-Qaeda subsequently stepped up attacks across the country, while Shia groups began deadly reprisals.
A suicide car bomb in the town of Baladruz killed at least 13 people on Thursday, according to local officials.
The bomb went off outside a car showroom in the town, which is 73km (45 miles) north-east of the capital Baghdad.
In Baghdad itself, a bomb killed four in the mainly Shia district of Shaab, while at least four civilians and two soldiers were killed in two separate attacks in the town of Latifiya, south of the capital.
The UN says 2013 was the deadliest year in Iraq since 2008, with at least 7,818 civilians and 1,050 members of the security forces killed in violent attacks.