It is placing "civilian hostages" near strategic locations and fighters, "effectively using tens of thousands of women, men and children as human shields", United Nations spokeswoman Ravina Shamdasani said in Geneva.
He said he and his family were only able to escape when a volley of airstrikes caused the fighters to scatter during the 40-kilometre forced march from their home in the town of Shura to Mosul.
Most were former members of the Iraqi security forces who were shot dead at the al-Ghazlani military base, the others were civilians shot in the head at another base after refusing to join the extremists, Shamdasani said.
The area is being cleared of homemade bombs and booby traps, he said.
Shamdasani reported ISIS forces have collected the citizens as an extra layer of protection, "to use them as human shields against the Iraqi forces advanc [ing] on Mosul".
"Just in the operations over the last week-and-a-half associated with Mosul, we estimate they've probably killed about 800-900 Islamic State fighters", Votel saod.
Despite the territorial gains, commanders have warned that securing Mosul could take weeks, if not months.
Iraqi soldiers and security forces and Kurdish peshmerga fighters, backed by US -led air strikes and support on the ground, already control Mosul's southern, eastern and northern flanks, and have advanced on those fronts for almost two weeks.
Colonel John Dorrian, a Baghdad-based military spokesman, said it was believed the militants meant to use the civilians as human shields.
Turkey's Foreign Minister Mevlut Cavusoglu said on Wednesday his country, which has troops deployed north of Mosul inside Iraqi territory, will take measures if there is an attack on Tal Afar.
Kurdish peshmerga fighters have fought with Iraqi government forces in a joint offensive to retake Mosul.
"We've seen. quite a dramatic increase in the numbers in the last few days, and they are now going into the newly set up camps", Mr Karl Schembri, regional media adviser for the Norwegian Refugee Council, told Agence France-Presse.
Trump also criticized former Army War College dean Jeff McCausland after the military expert said the candidate lacks a firm grasp of military strategy.
The World Health Organisation (WHO) said on Wednesday that of the 700,000 people expected to flee Mosul, some 200,000 would require emergency health services, including over 90,000 children needing vaccinations and 8,000 pregnant women.
More than 100 USA military personnel are embedded with them, advising commanders and helping direct coalition air strikes. Gen. Matthew C. Isler said the US -led coalition conducted "precision strikes" on vehicles that were unoccupied and far away from civilians.
Human Rights spokeswoman Ravina Shamdasani says IS, also known as ISIL, has abducted almost 8,000 families or more than 47,000 people and forced them at gunpoint into strategic IS locations.
Iraqi forces are holding positions about 6 kilometers (3.7 miles) from the eastern side of Mosul.