According to a Reuters report, troops are meeting fierce resistance as ISIS gangs retreat into the Old City, where street fighting is expected in the narrow alleyways and around the mosque where Islamic State declared its caliphate nearly three years ago.

A man who spoke to Reuters via telephone said he had found the mutilated body of a relative strung up from an electricity pole in the Tenek district along with three other young men caught trying to flee by the ISIS.

"Their appearance was shocking. We weren't able to get them down and they have been there for two days," said the relative on condition of anonymity.

According to the Kurdistan Region security council, the number of people killed by the ISIS on Monday and Tuesday was as high as 140.

 In the Old City, more than 40 civilians were killed when ISIS caught them trying to escape, said a resident of the Farouq district where the symbolic Nuri mosque is located.

Another resident of the Old City's Shahwan district said a family of six, including an elderly woman, had also been killed for the same reason.

During the major offensive launched for the liberation of the Mosul city from ISIS last year, eastern neighborhoods were entirely besieged and liberated by Iraqi troops. While the ISIS occupation continues in the western neighborhoods, it will reportedly take some time to take the city from ISIS.