Iraqi forces faced further pockets of resistance from Islamic State militants in Mosul's Old City yesterday, four days after the prime minister declared victory.

Iraqi army helicopters flew overhead and explosions could be heard, residents said, while videos of alleged revenge attacks against people detained during the retaking of Mosul underlined future security challenges.

"Three mortars landed on our district," a resident of Faysaliya, in east Mosul, just across the Tigris river, said by telephone.

A few hundred Islamic State insurgents swept into Mosul three years ago, imposed a reign of terror after the Iraqi army collapsed and declared a caliphate spanning Iraqi and Syrian territory seized in a shock offensive.

The victory of US-backed Iraq forces in Mosul marked the biggest defeat for Islamic State, which is under siege in the eastern Syrian city of Raqqa, its operational base.

Even though the Sunni Muslim group's caliphate is now crumbling, it is expected to revert to an insurgency and keep carrying out attacks in the Middle East and West.

Prime Minister Haider al-Abadi faces the challenge of preventing revenge killings that could create more instability, along with sectarian tensions and ethnic strife.