Iraq is going through its worst surge in violence since 2008, with near-daily attacks and relentless bombings blamed on hard-line Sunni insurgents. The surge followed a deadly crackdown by the Shiite-led government on a Sunni protest camp in northern Iraq in April.
September's toll pushed the total number of people killed since April to more than 5,000. Among the dead last month were 887 civilians while the rest were security forces.
The figure was slightly lower than the death toll in July, which was the highest since 2008 at 1,057, but underscored the rising violence after a long period of relative calm.
The report said the worst-affected part was the capital, Baghdad, where 418 people were killed in September. It said 2,133 people were wounded in last month's violence.
The U.N. representative in Baghdad said the report raised a stark alarm and called on Iraq's political rivals to come together.
"As terrorists continue to target Iraqis indiscriminately, I call upon all political leaders to strengthen their efforts to promote national dialogue and reconciliation," Nickolay Mladenov was quoted as saying in the report. "Political, religious and civil leaders as well as the security services must work together to end the bloodshed and ensure that all Iraqi citizens feel equally protected."
Hours earlier, al-Qaida's local franchise in Iraq claimed responsibility for Monday's string of car bombings that mostly targeted Shiite neighborhoods in Baghdad, killing 55 people. Sheik Mahmoud Abdel Rahman's grandson, Latif, and the baby's mother Hasnah Abdel Rasul, were among those killed in the wave of violence.
The baby was wrapped in a shroud with verses from Islam's holy book the Quran as Abdel Rahman carried it in a procession as his two loved ones were buried in the Shiite holy city of Najaf.
The Islamic State of Iraq and the Levant said the attacks were in retaliation to the "arrests, torturing and targeting of Sunnis" by the Shiite-led government and vowed more attacks.
"To Sunnis we say: The Islamic State, God willing, knows where, when and how to attack," the statement said. "The Mujahideen will not stay motionless."
The al-Qaida statement's authenticity could not be independently confirmed but it was posted on a website commonly used by militants and its style was consistent with earlier statements by the terror network.
Attacks continued on Tuesday, killing at least six people.
In the northern city of Tikrit, 130 kilometers (80 miles) north of the Iraqi capital, three suicide attackers tried to storm a building of the Interior Ministry's anti-explosives division, killing an officer and two policemen, officials said. Five other policemen were wounded.
In the town of Tarmiyah, about 50 kilometers (30 miles) north of Baghdad, a bomb went off next to a patrol of anti-al-Qaida Sunni militia, killing two and wounding three militiamen.
The pro-government militia, known as Sahwa, was established during the height of insurgency to fight al-Qaida along with U.S. and Iraqi forces. They are considered by al-Qaida as traitors.
In Baghdad, two gunmen on a motorcycle went on shooting spree in the southwestern Saydiyah neighborhood, killing one civilian and wounding six, another police officer said.
Three medical officials confirmed the causality figures. All officials spoke on condition of anonymity as they were not authorized to talk to the media.
Associated Press writer Qassim Abdul-Zahra contributed to this report.