The Islamic State group executed 741 civilians in the battle for the Iraqi city of Mosul, the UN said on Thursday, accusing the militant group of perpetrating “international crimes” during the nine-month military campaign.

A total of 2,521 civilians were killed, mostly by IS attacks, during the fight between IS and the internationally-backed Iraqi Security Forces (ISF) that ended in July, the UN 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 IS in 2014 and became the capital of the group's self-styled “caliphate” in the country.

Following IS'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.

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 UN said in a statement, noting that it was impossible to establish responsibility for the strikes “in almost all cases”.

The rights office urged the Iraqi government to invite the International Criminal Court to investigate the country's situation “as an immediate step”.

“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.