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 the Islamic State group declared its caliphate nearly three years ago.

A man who spoke 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.