ISIS fighters have abducted "tens of thousands" of men, women and children from areas around Mosul to use as human shields, the UN human rights office said on Friday.

The jihadist Sunni militants killed at least 232 people on Wednesday, including 190 former Iraqi security forces (ISF) and 40 civilians who refused to obey their orders, UN human rights spokeswoman Ravina Shamdasani told a news briefing.

"Credible reports suggest that ISIL has been forcing 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 operation began on the 17th of October to restore Iraqi government control over Mosul," Shamdasani told a briefing.

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

"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," Shamdasani said.

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

Up to 900 jihadis have been killed in the offensive to retake Mosul, the US military said on Thursday. It is thought up to 5,000 ISIS fighters were in Mosul ahead of the assault.

Despite significant territorial gains military commanders have said the campaign could take weeks if not months.

General Joseph Votel, the head of US Central Command, told AFP the offensive had inflicted a heavy toll on ISIS.

"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," he 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.

The UN High Commissioner for Human Rights, Zeid Ra'ad al-Hussein 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", said Shamdasani.

Additional reporting from Reuters.