IS militants have shot dead hundreds of fleeing refugees in Mosul
The civilians were "mown down" in their dozens as they tried to take refuge in a local hospital, AP reports.
"There were bullets falling on us like rain," said Riyadh Abdullah, who was caught in the onslaught.
Abdullah, 21, said he and his family, along with hundreds of neighbours, had tried to escape the city during a brief lull in the Iraqi army's shelling, when IS gunmen fired down on them from rooftops.
A separate witness to the massacre, Jassem Mohammed, said 12 members of his family had been killed in front of him in the attack.
"I was also hit but it wasn't that bad, I wasn't bleeding. Then, that afternoon, I was hit again by a sniper in the side and then I started to bleed," said Mohammed, aged-17.
"I was trying to lift my left leg and crawling on my hands. We crawled for three or four minutes and then rested for five minutes. And then crawled and crawled again. We kept crawling until we reached the army."
One American medic told reporters that his field clinic had treated 40 civilians for gunshot wounds following the sustained attack.
The commander of an Iraqi army brigade operating in the nearby district, Col. Hussein Mustafa, estimated that around 150 people were killed by IS militants.
Another victim, who lost 13 family members in the attack, said civilians had been forced to flee Mosul due to the Iraqi army’s sustained bombing of civilian areas.
"It's all because of the shelling," said Ahmed Najm, 27.