Islamic State militants have killed dozens of civilians attempting to flee Mosul in recent days, hanging several dead bodies from electricity poles as Iraqi forces fight to retake the city, witnesses said.

Troops are meeting fierce resistance as militants 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 militants.

"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.

The Kurdistan Region security council said the number of people killed by the militants on Monday and Tuesday was as high as 140.

In the Old City, more than 40 civilians were killed when the militants 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.     

A woman from the Yarmouk district said she had narrowly escaped death along with her husband and children after the militants caught them trying to flee among a group of around 30 people.

"They took our bags thinking there was gold or money in them and as they were busy checking the contents, we fled through the houses taking advantage of the pitch darkness," said the woman, who is now in an area under the control of the security forces.

"I fear those (families) who stayed in Daesh's grip met a terrible fate".