ISIL fighters in Mosul have deliberately targeted civilians who refused to join them as they retreated ahead of advancing Iraqi forces involved an offensive to recapture the city, an international rights group says.

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Mosul civilians were increasingly being caught in the crossfire with at least 19 killed and dozens wounded from the third week of November into the first week of December, a statement from Human Rights Watch (HRW) said.

The New York-based group said the deaths were caused by ISIL mortar or sniper fire, car bombs, roadside bombings, and direct attacks, as well as in air strikes by Iraqi forces and the US-led coalition.

The findings were based on interviews with more than 50 residents who had fled eastern Mosul, HRW said.

It cited instances of ISIL fighters telling residents that those who stayed behind were "unbelievers" and, therefore, valid targets beside the Iraqi and coalition forces.

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HRW warned that targeting civilians or using them as human shields is a war crime, and appealed to both sides to spare civilians.

"Civilians are being hit from all sides in Mosul," said Lama Fakih, deputy Middle East director at HRW, adding that ISIL's "atrocities do not absolve Iraqi forces and the international coalition from doing their utmost to protect civilians".

The Iraqi military launched a massive operation in October to retake Mosul, the country's second largest city and the group's last major urban bastion in Iraq. The troops' advance slowed once they pushed into more densely populated areas.

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ISIL captured Mosul in the summer of 2014 as part of a blitz that placed nearly one third of Iraq under its control. Since last year, ISIL has lost swaths of areas in western and northern Iraq.

Earlier this month, the Iraqi army denied accusations that it killed dozens of civilians when it launched air strikes on Mosul.

About 100,000 Iraqi soldiers, security forces, Kurdish Peshmerga forces and mainly Shia paramilitary forces are participating in the Mosul campaign.

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Although the army has made advances inside eastern Mosul, it says it is fighting in the toughest urban warfare imaginable - facing hundreds of suicide car bomb attacks, mortar barrages, sniper fire, and ambushes launched from a network of tunnels.

The city is still home to nearly one million people who are at risk of being caught up in the violence.

An estimated 650,000 people without potable water live in Mosul, and the UN has warned of a potential humanitarian crisis and refugee exodus.

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Source: News agencies