GENEVA (Reuters) - About 100,000 civilians remain trapped behind Islamic State lines in Mosul with a U.S.-backed government offensive to recapture the Iraqi city entering its ninth month, the United Nations refugee agency UNHCR said on Friday.

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Islamic State snipers are shooting at families trying to flee on foot or by boat across the Tigris River, it said.

“These civilians are basically held as human shields in the Old City,” said the UNHCR representative in Iraq, Bruno Geddo, referring to Mosul’s historic district where the militants are besieged by Iraqi government forces.

“There is hardly any food, water, electricity, fuel. These civilians are living in an increasingly worsening situation of penury and panic because they are surrounded by fighting.”

The offensive to retake Mosul, Islamic State’s de facto capital in Iraq, started on Oct. 17 with air and ground support from a U.S.-led international coalition.

Iraqi government forces regained eastern Mosul in January, then a month later began the offensive on the western side that includes the Old City.

The Old City “is a very dense labyrinth, a maze of narrow alleyways where fighting will have to be done on foot, house by house,” said Geddo.

“ISIS (Islamic State) snipers continue to aim at people trying to flee because there is this long-standing policy of executing people trying to flee the territory of the caliphate,” he said.

The fall of Mosul would, in effect, mark the end of the Iraqi half of the “caliphate” that Islamic State leader Abu Bakr al-Baghdadi declared in a speech from an historic mosque in the Old City three years ago, covering parts of Iraq and Syria.

Moscow said on Friday its forces may have killed Baghdadi in an air strike in Syria last month, but Washington said it could not corroborate the death and Western and Iraqi officials were skeptical.

About 200,000 people were estimated to be trapped behind Islamic State lines in Mosul in May, but the number has declined as government forces have thrust further into the city.

About 800,000 people, more than a third of the pre-war population of the northern Iraqi city, have fled, seeking refuge with friends and relatives or in camps. UNHCR has provided many with shelter, food and other necessities.

Geddo voiced deep concern about “collective punishment” of families whose relative may have been an IS fighter.

“Collective punishment means in a deeply tribal society that you see evictions, destruction of property, confiscation of property for families perceived as being associated with ISIS because one family member might have been having that link.

“This is a very critical point for the future of Iraq. Because it is essential to uphold the rule of law, to pursue those who committed crimes through the court system, the judicial system, rather than applying tribal custom,” he said.