Hundreds of civilians are being killed and injured in fighting in the Old City of Mosul, where the government is trying to dislodge the Islamic State from its last major stronghold in the country, a United Nations official said Saturday.

“Fighting is very intense in the Old City and civilians are at extreme, almost unimaginable risk. There are reports that thousands, maybe even tens of thousands, of people are being held as human shields,” said Lise Grande, the U.N. humanitarian coordinator for Iraq.

“Hundreds of civilians, including children, are being shot,” she said in a statement.

Grande estimated that 100,000 to 150,000 civilians are still trapped inside the Old City, a week after Iraqi forces, backed by U.S,-led air power, started an offensive to drive the Islamic State from the last area under its control in Mosul.

Government forces have since reported slow advances against militants in the Old City in western Mosul, known for its high population density and narrow alleyways.

On Wednesday, Islamic State extremists blew up the Old City’s ancient al-Nuri Mosque, where the radical group’s leader, Abu Bakr al-Baghdadi, made his only public appearance in 2014.

In February, Iraqi forces started a U.S.-backed onslaught to wrest back the western section of Mosul from the Islamic State, almost a month after they recaptured the eastern part of the city.

The Islamist extremists seized the northern Iraqi city, the country’s second largest, in 2014.