QAYARA AIR BASE, Iraq — The Islamic State group appears to be using tens of thousands of people as “human shields” in and around Mosul, where Iraqi forces are waging a large-scale offensive aimed at retaking the country’s second largest city, the U.N. human rights office said Friday.

It has received reports of more than 200 people being killed for refusing to comply with IS orders or previously belonging to Iraqi security forces. It said “credible reports” suggest IS has been forcing tens of thousands from their homes in districts around Mosul.

“ISIL’s depraved, cowardly strategy is to attempt to use the presence of civilian hostages to render certain points, areas or military forces immune from military operations, effectively using tens of thousands of women, men and children as human shields,” spokeswoman Ravina Shamdasani said in Geneva, using an alternative acronym for the extremist group.

The extremist group has massacred perceived opponents on several occasions, and is widely believed to be rooting out anyone who could potentially rise up against it, focusing on Iraqis with military training or past links to security forces.

The U.N. office said civilians from across the region south of Mosul were being herded into Hamam al-Alil, a militant-held town where the population has more than doubled to 60,000 since the forced displacement began.

There the militants separated former members of the security forces from women and children, and took both groups onward to Mosul, the U.N. agency said. It said 190 former security forces were killed Wednesday at the Ghazlani military base on the southern edge of Mosul, and 42 civilians were killed at another base for refusing to join IS. Another 24 people were reportedly shot dead on Tuesday.

Iraq launched a massive operation on Oct. 17 aimed at retaking Mosul, which fell to IS in a matter of days in the summer of 2014. Iraqi forces are advancing from several directions, but are still well outside the city itself.

The U.S. military, which is providing airstrikes and ground support for the operation, said it tried to disrupt the forced displacement of civilians south of Mosul earlier this week by striking militant vehicles being used in the operation.