At least 1,075 people were killed and 658 injured in the country in the 17 days from June 5 and 22, Rupert Colville, a spokesman for the UN human rights office told reporters in Geneva.
He said the numbers "should be viewed very much as a minimum."
At least 757 civilians were killed and another 599 injured in the provinces of Nineveh, Diyala and Salah al-Din, he said.
A number of the deaths were due to "verified summary executions and extra-judicial killings of civilians, police, and soldiers who were hors combat."
Also Read
Militants, led by jihadists from the Islamic State of Iraq and the Levant, or ISIL, have since the beginning of June overrun major areas of five provinces and driven to within less than 100 kilometres of Baghdad.
In addition to the killings, Colville said widespread abductions were reportedly continuing across the affected provinces.
Many foreigners are among those abducted, including the 48 Turkish citizens snatched from Ankara's consulate when ISIL captured Mosul, and around 40 Indian nationals working for an Iraqi construction company who were kidnapped on June 18, according to India's foreign ministry.