"Last Wednesday 232 civilians were reportedly shot to death. Of these there were 190 former Iraqi Security Forces officers," rights office spokeswoman Ravina Shamdasani told reporters in Geneva.
"These reports have been corroborated to the extent possible," she added, stressing that the number of people killed in recent days could be higher.
Shamdasani said the execution-style killings came as IS was pushing forward with its strategy of forcing people living near the northern Iraqi city into Mosul, the group's last bastion in Iraq.
"ISIL has been forcing tens of thousands of people from their homes in some districts around Mosul," Shamdasani said.
Those being executed have included civilians who have refused to comply with the relocation orders and those who previously worked for the government security services.