One man said he had found a relative’s mutilated body displaced in the Iraqi city’s Tenek district alongside three others who were caught trying to escape.
“Their appearance was shocking,” he told Reuters. “We weren’t able to get them down and they have been there for two days.”
Another resident of the Old City’s Shahwan district said a family of six, including an elderly woman, had also been murdered by jihadis.
The woman said she had narrowly escaped the same fate herself, when Isis fighters intercepted her as she tried to cross frontlines with 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,“ she added, after reaching safety in areas under Iraqi government control.
“I fear those families who stayed in Daesh’s grip met a terrible fate.”
A resident of the Farouq district said more than 40 civilians were killed for attempting to flee, while the Kurdistan Region security council put the toll at 140 for Monday and Tuesday alone.
Isis has shot men, women and children attempting to leave its waning territories across Iraq and Syria, attempting to use families as human shields.
US-led coalition air strikes killed up to 200 civilians in Mosul last month after militants reportedly forced them inside a cluster of buildings, then stationed its snipers on top.
Announcing an investigation into the incident, a spokesperson for the US-led Operation Inherent Resolve said the group was using “inhuman tactics terrorising civilians, using human shields, and fighting from protected sites such as schools, hospitals, religious sites and civilian neighbourhoods”.
The US-led coalition has confirmed the deaths of 229 civilians in its air strikes in Iraq and Syria but monitors say the real figure is far higher and casualties in Isis-controlled territory are difficult to verify.
There have also been allegations of war crimes, summary executions and torture by troops and militias fighting Isis.
One video circulating on social media appeared to show members of a Shia Iranian militia shooting two civilians.
The blindfolded men, who were not wearing military fatigues, had their arms bound and were forced to stand on the edge of a ditch before being shot in the back.
Iraqi government forces briefly paused the Mosul offensive last month because of the rate of civilian casualties, which has worsened since troops crossed into densely populated western districts that are still home to an estimated 400,000 people.
Progress has slowed in the narrow lanes of the Old City, which Isis had prepared by making tunnels between buildings, screening streets from view with sheets and planting explosives.
More than 302,000 people have fled Mosul since the start of the offensive, with the International Organisation for Migration saying 30,000 people were displaced last week alone.
Isis leader Abu Bakr al-Baghdadi announced the formation of the group’s self-declared “caliphate” from Mosul’s historic al-Nuri mosque in 2014.
The so-called Islamic State’s de-facto capital is the city of Raqqa in neighbouring Syria, which has been isolated by Kurdish-led forces in preparation for an offensive.
Concerns are already rising over civilian casualties in that operation, following an alleged massacre in the nearby village of Hunaida.
The UK-based Syrian Observatory for Human Rights said at least 15 people, including four children, were killed by US-led coalition air strikes, which activists said hit an internet café.
The anti-Isis Raqqa is Being Slaughtered Silently group said a boat carrying around 40 civilians attempting to cross the Euphrates river to reach areas controlled by the Syrian Democratic Forces was also targeted.