Killings meant "to terrorise" as fighters accused of surrounding themselves with civilians in Hamam al-Alil village.
The Iraqi army was trying on Thursday to reach a town south of Mosul where ISIL has reportedly executed dozens of civilians to deter any support for the military offensive to recapture the group’s stronghold.
Eleven days into the biggest ground operation in Iraq since the 2003 US-led invasion, army and federal police units were fighting off sniper fire and suicide car bombs south of Hamam al-Alil - the site of the reported executions.
Islamic State of Iraq and the Levant fighters shot dead dozens of prisoners there, most of them former members of the Iraqi police and army, taken from villages the group has been forced to abandon as troops advanced.
A Reuters news agency correspondent met relatives of hostages south of Mosul. One was a police officer who returned to see the family he left behind when his village fell under ISIL control two years ago.
"I’m afraid they will keep pulling them back from village to village until they get to Mosul. And then they will disappear," he said, asking not be identified to protect family members still in the hands of the fighters.
The executions were meant "to terrorise the others, those who are in Mosul in particular", and also to get rid of the prisoners, said Abdul Rahman al-Waggaa, a member of the Nineveh provincial council. Some of the families of those executed are also held in Hamam al-Alil, he said.
UN human rights spokesman Rupert Colville said on Tuesday that ISIL had reportedly killed scores of people around Mosul in the last week.
Reuters also spoke to a woman and an elderly man who were among a group of families forced to march two to three days to reach Mosul from the villages of Safiya and Ellezaga, about 30km and 50km, respectively, to the south.
Children and the elderly were released when they arrived in Mosul on Tuesday and told to stay with relatives, they said, speaking on the phone from the city’s edges.
A resident of Mosul, Rayyan, said he saw the families when they arrived in the city, "their bare feet bleeding and covered with dust as if coming from under the rubble.
"We cried when we saw them," he said.
Local officials, activists, and a resident of Qayyara district told human rights group Amnesty International that civilians were kept in schools, homes, and other locations near ISIL fighters in Hamam al-Alil after being forced to move from their homes.
ISIL deliberately prevented them from fleeing areas of conflict and fighters embedded themselves within the civilian population.
"Using civilians to shield yourself from attack is a war crime. But even in cases when IS fighters are holding civilians as human shields, this does not absolve Iraqi and coalition forces from the obligation to take their presence into account, take all feasible precautions to minimize harm to civilians, and avoid launching attacks that could cause disproportionate harm to civilians," said Amnesty’s Lynn Maalouf in a statement.
Iraqi and Kurdish forces are recapturing territory as part of the offensive to retake Mosul from Islamic State of Iraq and the Levant, which is known by its opponents in Arabic as Daesh.
On Thursday, Kurdish Peshmerga forces - fighting alongside Iraqi soldiers and militiamen - seized the northern Iraqi village of Fadiliya, which lies about 4km away from Mosul.
Residents had hung up white flags on their homes, hoping to avoid being targeted by air strikes.
Children joyfully ran through the streets singing songs and chanting "Peshmerga", "Peshmerga", "Peshmerga".
One fighter named Ahmed arrived at his home to see his loved ones for the first time in two years.
His sister stood in the doorway and wept and screamed while looking up to the sky. "Daesh ruined our lives. Look what they have done to us," she said.
As more and more people came out of their homes to taste freedom that had been unthinkable for so long, Khalid Abdel Hafiz tried to hold back tears as he recalled a knock on his door one week ago. ISIL fighters took away his 19-year-old son, Ihsan, along with 14 other young men.
"They just showed up and said ’We have security orders from high up’," he said. "I have no information about Ihsan. I am resigned to the fact I will never see him again."