A new United Nations report has found that the Islamic State group committed serious "international crimes" such as using civilians as human shields during the Iraqi military's nine-month campaign to liberate the city of Mosul.

The report published Thursday by the U.N. assistance Mission for Iraq and the Office of the U.N. High Commissioner for Human Rights is based on witness testimony. It documents the mass abduction of civilians, the intentional shelling of civilian residences and indiscriminate targeting of civilians trying to flee the city.

Mosul fell to IS in a matter of days in 2014, but the campaign to retake the city lasted nearly nine months. The fight, closely backed by airstrikes from the U.S.-led coalition, brought an end to the extremists' so-called territorial caliphate.