Near Bashiqa, Iraq (CNN) The reports are disturbing: Tens of thousands of men, women and children snatched from their homes and forced into the center of Mosul as the battle intensifies to drive ISIS out of Iraq's second-largest city.

The intent is sinister: Using civilians as human shields is ISIS's attempt to hold onto the city, the jewel of its self-proclaimed caliphate.

No one expected the militants to surrender Mosul without a hellish fight. But for Iraqis still living under ISIS control in the city and surrounding areas, every option now is grim.

They can try to flee but be branded by ISIS as "apostates," for which the penalty is death. Or they can potentially be seen as ISIS sympathizers in a new place and risk becoming victims of revenge crimes.

They can try to hold out and survive the fierce fighting ahead. Or they can be forced further into the heart of Mosul and serve as human shields.

It's a horrifying prospect: be killed by ISIS or be killed in the attacks to defeat ISIS.

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'Depraved, cowardly strategy'

From the beginning, it was feared innocent people would pay the heaviest price in what could be the largest ground operation in Iraq since the US-led invasion in 2003. This is true in any war, but Iraqis who have already suffered for more than two years under the brutal reign of the Islamic State are now being subjected to a new set of horrors.

Human shields are a way for ISIS to ensure immunity from fire, said the spokeswoman for the United Nations High Commissioner for Human Rights. The militant group is also known as IS, ISIL and its Arabic acronym, Daesh.

"ISIL's depraved, cowardly strategy is to try to use the presence of civilian hostages to render certain points, areas or military forces immune from military action, essentially using tens of thousands of men, women and children as human shields," said the UN agency's spokeswoman, Ravina Shamdasani.

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ISIS has abducted nearly 8,000 families from districts surrounding Mosul, the UN agency said, based on reports it has received. They were forced out at gunpoint and if they refused, they were killed.

Thousands of men, women and children were forced to walk barefoot alongside ISIS fighters as they retreated toward Mosul.

In Hamam al-Alil, an ISIS stronghold south of Mosul, the population has spiked to 60,000 from the previous 23,000, the United Nations said. Iraqi forces said they hope to storm the town soon but the high number of civilians is a definite concern.

Kurdish Peshmerga forces have the same concern in the towns and villages north and east of Mosul that they have been clearing.

"We are taking it slowly," said Kurdish Peshmerga Col. Abdullah Tato, whose soldiers encircled the ISIS-held town of Bashiqa, a few miles northeast of Mosul.

"We have to be careful," Tato said. "Our biggest challenge are the civilians. We have to be patient in order to save civilian lives."

A war crime: Amnesty

Iraqis who managed to escape the clutches of ISIS have told human rights groups about how the hardline fighters embedded themselves within the civilian population in residential areas or forced them to move into areas under their control.

"Using civilians to shield yourself from attack is a war crime," Lynn Maalouf of Amnesty International said in a recent statement.

"But even in cases when IS fighters are holding civilians as human shields, this does not absolve Iraqi and coalition forces from the obligation to take their presence into account, take all feasible precautions to minimize harm to civilians, and avoid launching attacks that could cause disproportionate harm to civilians," Maalouf said.

Many Iraqi families have been displaced by the ongoing operation to retake the city of Mosul from ISIS.

Urban warfare, including the kind of street-to-street fighting that is going on now in Iraq, is not prohibited under international law. But it makes it very clear that every side has an obligation to minimize potential harm to civilians.

That makes the use of human shields -- the deliberate placement of people at legitimate target areas in order to shield those areas from attack -- a violation of international humanitarian law as defined by the Geneva Conventions.

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Grim historic precedents

It was perhaps Iraq's Saddam Hussein who made the term "human shield" a household term. The former Iraqi dictator used thousands of Iraqis as well as foreigners to try and deter international military strikes against him, according to a 2003 CIA report.

He held more than 800 Western, Japanese and Kuwaiti nationals in Iraq and Kuwait to safeguard his regime after he invaded Kuwait in 1990. Many have reported being psychologically scarred by the trauma, their lives marred by depression, divorce and suicidal notions.

Later, in 1997, Hussein used Iraqis as shields after he refused UN inspections of key institutions and sites.

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As gruesome as the tactic of using human shields is, it is hardly limited to recent conflicts. The ruthless Mongol Genghis Khan used unskilled laborers as human shields. During the American Civil War, the Confederate Army jailed 50 Union officers in Charleston, South Carolina, and used them to thwart Union artillery from firing on the city.

Now, in Iraq, thousands of people could lose their lives because ISIS is forcing them into the action.

The war is raging on the rugged plains of Nineveh; in the houses, schools and mosques of towns and villages. Soon fighting will erupt in the heart of Mosul, a far more densely populated place. No one knows exactly how many people are still living there but it could be more than 1 million.

The city has been a prison for its residents for more than two years under ISIS. Now, with the extremist group amassing people around its fighters, Mosul could tragically become a graveyard.