The U.N. Assistance Mission in Iraq said in a statement 1,045 Iraqis were killed and another 2,397 were wounded in May.
"That is a sad record," said Martin Kobler, Special Representative of the United Nations Secretary-General for Iraq. "Iraqi political leaders must act immediately to stop this intolerable bloodshed."
Of the 963 civilians who were killed in May, 181 were police officers. The Iraqi Security Forces lost 82 members.
Much of the violence involved from sectarian fighting between Shiite and minority Sunni factions. The BBC said an active rumor mill in and around Baghdad was fueling the anxiety in the capital.
Analysts say Sunni factions with ties to al-Qaida have been trying to provoke a civil war between the Shiites and Sunni.
There were concerns the violence in neighboring Syria was having a spillover effect in Iraq, the BBC said.