The Daesh terrorist group executed 741 civilians in the battle for the Iraqi city of Mosul, the U.N. said Thursday, accusing the terrorists of perpetrating "international crimes" during the nine-month military campaign.

A total of 2,521 civilians were killed, mostly by Daesh attacks, during the fight between Daesh and the internationally-backed Iraqi Security Forces (ISF) that ended in July, the U.N. rights office said in a report.

"Those responsible must answer for their heinous crimes", the United Nations High Commissioner for Human Rights, Zeid Ra'ad Al Hussein, said in a statement.

Mosul, Iraq's second city was captured by Daesh in 2014 and became the capital of the group's self-styled "caliphate" in the country. Following Daesh's defeat in the city, the rights office said it had compiled witness testimony documenting "mass abductions of civilians, the use of thousands as human shields, the intentional shelling of civilian residences, and indiscriminate targeting of civilians trying to flee the city". More than 800,000 people were displaced by the fighting, the report said.

The rights office also called for investigations into alleged violations committed by the ISF and their allies, including militia groups. It urged Iraq to accept the jurisdiction of the International Criminal Court to prosecute atrocities such as genocide, crimes against humanity and war crimes, and said ensuring justice was key to rebuilding trust and to lasting reconciliation in the country.

The report "recorded 461 civilian deaths as a result of airstrikes during the most intensive phase of the ISF-led offensive from 19 February", the U.N. said in a statement, noting that it was impossible to establish responsibility for the strikes "in almost all cases."

"By prosecuting those responsible for ‘international crimes' in Mosul the Iraqi authorities would be sending a message to the people of Iraq who have suffered, no matter when or where, that justice is eventually delivered," the rights office said.

Losing Raqqa has been a huge blow for Daesh, which has steadily lost territory in Iraq and Syria, including Iraq's second largest city of Mosul a few months ago. The group declared the city on the banks of the Euphrates River, which it seized from other Syrian opposition fighters in early 2014, to be the capital of its self-declared "caliphate," transforming the once vibrant metropolis into the epicenter of its brutal rule where opponents were beheaded and terror plots were planned. The terrorist group has also suffered setbacks in the eastern Syrian region of Deir ez-Zor, where Russian-backed regime forces retook swathes of territory, further reducing a "caliphate" that three years ago was roughly the size of Britain.