A further 1,370 Iraqis were wounded and 600,000 people forced to flee as Islamic State militants, who have grabbed large areas of territory since June, pushed into land controlled by Kurdish troops and targeted religious minorities.
"Thousands continue to be targeted and killed by ISIL (Islamic State) and associated armed groups simply on account of their ethnic or religious background. The true cost of this human tragedy is staggering," the UN representative in Iraq, Nickolay Mladenov, said.
The UN said the casualty figures could be far higher but it could not get independent verification of reports of hundreds of incidents in areas under Islamic State's control.
Violence killed 1,737 people, mostly civilians, in Iraq in July, and 2,400 in June, the UN data showed.
The fighting carried on Monday as 18 people were killed and more than 50 wounded when two parked car bombs went off in two mainly Shi'ite districts of Baghdad, police and medical officials said.
Both Islamist fighters and Iraqi government forces have committed atrocities in the three months of fighting, senior UN officials said in Geneva during an emergency debate on the conflict on Monday.