According to the United Nations Assistance Mission for Iraq (UNAMI), at least 856 civilians and 417 security forces members were killed and more than 2,000 were injured.
At least 1,119 were killed the previous month, UNAMI said.
Iraqi government statistics compiled by the health, interior and defence ministries put the number of people killed in October at 1,725 civilians and members of the security forces, with more than 2,300 wounded.
"I express my deep outrage for the ongoing kidnappings and murders of scores of people from all segments, ethnicities or religions of the Iraqi population," UN Iraq envoy Nickolay Mladenov said in a statement.
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"Justice and accountability for those responsible for these mass executions, killings, disappearances, and displacement must be ensured," he said.
The UNAMI casualty figures exclude Anbar province, west of Baghdad, where the Islamic State (IS) jihadist group has dealt pro-government forces a string of setbacks in recent weeks, sparking warnings that the whole province could fall.
IS spearheaded a militant offensive in June that overran Iraq's second city Mosul and then swept through much of the country's Sunni Arab heartland.
While security forces and allied fighters, now backed by US-led coalition air strikes, have made some advances, major areas remain outside their control, including several cities that will be especially difficult to retake.
Many deaths are believed to go unreported and hundreds of people are still considered missing across the country.