The UN human rights team in Iraq reports at least 757 civilians died in Nineveh, Diyala and Salahuddin provinces between 5 and 22 June.
It adds that at least another 318 people were killed during the same time in Baghdad and areas in southern Iraq.
UN spokesman Rupert Colville said the figure “should be viewed very much as a minimum”.
He added that it included some verified summary executions and extra-judicial killings of civilians, police officers and soldiers who had stopped fighting.
This month has seen insurgents, spearheaded by Islamists fighting under the banner of the Islamic State in Iraq and the Levant (Isis), overrunning a swathe of territory in the north and west including the second-biggest city, Mosul.
In its report on Iraqi deaths in June, the UN cites the examples of 15 Shia civilians who were abducted from the village of Pirwajili in Salahuddin province and killed, and reports that a further 45 unidentified bodies were found on the banks of the Tigris river.
In addition, UN human rights officers in Iraq have confirmed reports of summary executions carried out by Iraqi security forces as they withdrew from the Nineveh Operations Command HQ in the city of Mosul as it was overrun by Isis fighters.