Residents of Mosul said Daesh was using civilians as human shields as Iraqi and Kurdish forces captured outlying villages in their advance on the jihadists' stronghold.
The leader of Daesh was reported to be among thousands of hardline militants still in the city, suggesting the group would go to great lengths to repel the coalition.
With attacking forces still between 20 and 50 kilometres away, residents reached by telephone said more than 100 families had started moving from southern and eastern suburbs most exposed to the offensive to more central parts of the city.
Daesh militants were preventing people fleeing Mosul, they said, and one said they directed some towards buildings they had recently used themselves.
"It's quite clear Daesh has started to use civilians as human shields by allowing families to stay in buildings likely to be targeted by airstrikes," said Abu Mahir, who lives near the city's university.
Like other residents contacted in the city, he refused to give his full name, but Abdul Rahman Waggaa, a member of the exiled Provincial Council of Nineveh of which Mosul is the capital, corroborated his account to Reuters.
Around 1.3 million people still live in Mosul and the International Organisation for Migration (IOM) said Daesh may use tens of thousands of residents as human shields to hold onto their last city stronghold in Iraq.
The IOM said there was a likelihood of chemical attacks by Daesh, who had used such weapons previously against Iraqi Kurdish forces.
Iraqi Prime Minister Haider al-Abadi said safe routes had been secured for civilians who wanted to leave Mosul. Syria meanwhile accused the US-led coalition of planning to allow Daesh militants to flee across the border into Syria.
The fall of Mosul would signal the defeat of the ultra-hardline Sunni jihadists in Iraq but could also lead to land grabs and sectarian bloodletting between groups which fought one another after the 2003 overthrow of Saddam Hussein.