Gunmen attacked several security checkpoints in the Iraqi city of Samarra on Thursday morning, sparking hours of deadly fighting with Iraqi security forces, police officials in Samarra said.
At least 23 people were killed and 33 others were injured in the fighting, health officials there said.
The attackers are suspected to be members of the Islamic State in Iraq and Syria, an al Qaeda splinter group known by the acronym ISIS, police said.
Samarra is a predominantly Sunni Muslim city, about 100 kilometers (62 miles) north of Baghdad.
Iraq has been plagued by sectarian and political violence, some of it fueled by widespread discontent among Sunni Muslims, who say they are marginalized by the Shiite Muslim-led government and unfairly targeted by brutal security tactics.
ISIS has also fought in Iraq and neighboring Syria for a home for its brand of hard-line Islamism.
Last year was the deadliest in Iraq since 2008, with more than 8,800 people killed, most of them civilians, according to the United Nations.