Casualty figures released by the UN mission in Baghdad suggest 1,057 Iraqis were killed in July, making it the most violent month in years.
At least 4,137 civilians have been killed and 9,865 injured so far this year, with Baghdad province worst hit.
"The impact of violence on civilians remains disturbingly high," UN Iraq representative Gyorgy Busztin said.
"Iraq's political leaders must take immediate and decisive action to stop the senseless bloodshed," he warned.
A wave of violence has swept Iraq in the past six months, with Sunni Islamist militant groups targeting Shia districts. In July there was a series of bombings in streets, cafes and mosques. Militants also broke into two prisons, setting inmates free.
Mr Busztin - in Baghdad as acting special representative for UN Secretary General Ban Ki-moon - said the toll for July was the highest for more than five years.
He characterised the earlier period - the peak of the anti-US insurgency in 2006-07 - as one filled with the "blind rage of sectarian strife".
Iraq's government must "prevent these dark days from returning", Mr Busztin said.
There were 928 civilians killed in July (including 204 civilian police), and 2,109 civilians were injured (including 338 civilian police). A further 129 members of the Iraqi security forces were killed and 217 injured.
Baghdad was the worst-affected area with 957 civilian casualties (238 killed and 719 injured), followed by Salahuddin, Ninewa, Diyala, Kirkuk and Anbar.
Many of these provinces are dominated by members of the country's Sunni minority, many of whom resent the Shia-led government that took power after Saddam Hussein was toppled in 2003.