TIKRIT, Iraq Nearly 900 civilians were killed across Iraq in September as sectarian violence worsened, raising the death toll for 2013 to more than last year's total, the United Nations said on Tuesday.
Most of the bloodshed was instigated by Sunni Islamist militants. In addition to 887 civilians, 92 members of the security forces were killed.
September's toll brought the number of people killed so far this year to 5,740, exceeding the toll for all of 2012, when the first yearly increase in civilian deaths was recorded since 2009 following the withdrawal of U.S. troops in December 2011.
Overall, Baghdad was the worst-affected governorate.
"As terrorists continue to target Iraqis indiscriminately, I call upon all political leaders to strengthen their efforts to promote national dialogue and reconciliation," the U.N. envoy to Iraq, Nickolay Mladenov.
After reaching a climax in 2006-07, violence in Iraq eased when Sunni tribesmen banded together and found common cause with U.S. troops to rout al Qaeda, forcing it underground.
But the group has been reinvigorated this year by growing resentment of Iraq's Shi'ite-led government, which minority Sunnis accuse of marginalizing them since the overthrow of Saddam Hussein in 2003.
The civil war in neighboring Syria, where mainly Sunni rebels are fighting to topple a leader backed by Shi'ite Iran, has also put strain on Iraq's delicate intercommunal balance.
In the latest violence, a suicide bomber drove up to a police station in Tikrit and blew himself up, killing at least two guards.
Two gunmen disguised in police uniform then managed to enter the building, home to a unit specializing in bomb defusing, and clashed with staff inside for nearly one hour. They killed three policemen, including a colonel, before they were shot down, police said.
"I was inside the office when a powerful blast shook the building," said a police officer contacted during the gunfight. "I heard gunshots and screams inside and now I'm locked in my room with my gun loaded and ready to shoot."
No group immediately claimed responsibility for the attack, but suicide bombings are the hallmark of al Qaeda's Iraqi affiliate, which merged with its Syrian counterpart earlier this year to form the Islamic State of Iraq and the Levant (ISIL).
On Tuesday, the combined group claimed responsibility for a series of car bombs targeting Shi'ites in Baghdad that killed 54 people a day earlier.
In a statement posted on militant Internet forums, ISIL said it had carried out the bombings in revenge for what it described as a "campaign of torture, displacement, detainment and liquidation" of Sunnis by the Shi'ite-led government.
"The mujahideen (holy warriors) will not stand with their hands tied while the despicable project of Iran shows its ugly face in Iraq and the Levant," ISIL was quoted as saying by the SITE monitoring group.
(Reporting by Ghazwan Hassan; Writing by Ahmed Rasheed; Editing by Isabel Coles and Angus MacSwan)