ISIS Using Tens of Thousands as 'Human Shields' as Iraqi-Led Forces Advance on Mosul

— -- The Islamic State group has abducted tens of thousands of civilians to use as “human shields” as Iraqi-led forces close in on Mosul to liberate the country’s second largest city, the United Nations said Friday.

"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," said Ravina Shamdasani, spokeswoman for the office of the United Nations High Commissioner for Human Rights told reporters in Geneva.

The U.N. human rights office has in addition received reports that the group also known as ISIS or ISIL executed 256 people, mostly former officers in Iraqi forces, on Tuesday and Wednesday.

Meanwhile, Iraqi special forces that are spearheading the large-scale operation to free Mosul from ISIS have reached Tub Zawa, a populated village just five miles from Mosul.

But Iraqi commanders said the elite counterterrorism troops known as the golden division have halted their advance and, as of Friday morning, were awaiting U.S.-backed forces before converging on Mosul from the south and northeast. Iraqi Shiite militias will also be joining the fight west of the ISIS-held city.

The U.S. military said Friday that Iraqi forces have liberated 40 villages from ISIS since the operation began a week ago to drive out the militants from Mosul. However, most of the villages have been sparsely populated farming communities on the outskirts of Iraq’s second largest city.

Iraqi Army Maj. Gen. Najim al-Jabori said forces south of Mosul on Wednesday pushed ISIS fighters out of the town of Staff al-Tut in the Tigris River valley and were just 20 miles from the besieged city. And on Thursday, Iraqi forces retaking villages freed more than 1,000 civilians who had been detained by ISIS.

While clearing out territory liberated from ISIS, Iraqi special forces have discovered extensive tunneling networks that ISIS militants apparently used to dodge U.S.-led coalition airstrikes. The extremist group also rigged many homes and buildings with explosives before retreating from these areas, according to Iraqi commanders.

On Oct. 17, about 18,000 Iraqi forces, 10,000 Kurdish forces known as peshmerga and a few thousand Iraqi federal police launched the operation to free Mosul from more than two years of ISIS control. Roughly 100 American advisers are also involved in the mission, which is divided into two fronts -- one west of the Great Zab River and the other just north of Qayyarah.

As the fighting intensifies, many civilians are fleeing the areas under siege. The United Nations Children’s Fund (UNICEF) has warned that an “unprecedented humanitarian crisis” looms as up to a million civilians are expected to flee Mosul in the coming days and weeks.

“The challenges in this scenario are unprecedented. We don’t often have up to one million people potentially on the move; it’s very rare in scale and size,” UNICEF regional emergency adviser Bastien Vigneau said last week.

The Associated Press contributed to this report.