Islamic State forces in Iraq have abducted tens of thousands of men, women and children from areas around Mosul and are using them as "human shields" in strategic sites in the city as Iraqi troops advance, the U.N. human rights office said on Friday.

The hardline Sunni militants, known as ISIL, killed at least 232 people on Wednesday, including 190 former Iraqi troops and 42 civilians who refused to obey their orders, it said.

"ISIL's depraved, cowardly strategy is to attempt to use the presence of civilians 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," said Zeid Ra'ad al Hussein, U.N. High Commissioner for Human Rights.

Using human shields is prohibited under international law, he said in a statement.

Earlier, U.N. human rights spokeswoman Ravina Shamdasani cited "credible reports" that ISIL militants have forced "tens of thousands of people from their homes in sub-districts around Mosul and have forcibly relocated numbers of civilians inside the city itself" since the government assault began on Oct. 17th.

This was to "use them as human shields, to be able to keep them close to military installations...to try to frustrate the military operation against them," she told a briefing.

Nearly 8,000 families, of roughly six people each, were abducted in four sub-districts including Shura, she said.

"Many of those who refused to comply were shot on the spot".

The reports, from people who have fled and aid groups, have been corroborated by the United Nations, Shamdasani said.

Iranian-backed Iraqi Shi'ite paramilitary groups are about to launch an offensive on Islamic State positions west of Mosul, assisting in the military campaign to take back the city, a spokesman said on Friday.

Zeid has voiced deep concern at reports that some individuals in the areas south of Mosul have "embarked on revenge killings and have vowed on television that there would be 'eye-for-eye' revenge against those who sided with ISIL", Shamdasani said.

Some villagers have been prevented from returning to their villages due to their perceived support of ISIL, she said.

Government screening is in place to check people fleeing Mosul but the process must be carried out in humane conditions respecting international standards.

"Captured ISIL fighters and those perceived to have supported them must be treated fully in accordance with international law and held accountable for their crimes by properly constituted tribunals," Zeid said.