A displaced Iraqi woman, who fled the Islamic State stronghold of Mosul, carries her baby in the Mithaq district of eastern Mosul, Iraq, January 3, 2017. [Photo/Agencies]

Security forces have retaken about a quarter of Mosul since October but, against expectations and despite severe shortages of food and water, most residents have stayed put until now.

More than 125,000 people have been displaced out of a population of roughly 1.5 million, but the numbers have increased by nearly 50 percent to 2,300 daily from 1,600 over the last few days, the UNrefugee agency said.

The humanitarian situation was "dire", with food stockpiles dwindling and the price of staples spiralling, boreholes drying up or turning brackish from over-use and camps and emergency sites to the south and east reaching maximum capacity, it said.

Most of the fleeing civilians are from the eastern districts but people from the besieged west, still under the militants' control, are increasingly attempting to escape, scaling bridges bombed by the coalition and crossing the Tigris by boat.

An Iraqi victory in Mosul would probably spell the end for Islamic State's self-styled caliphate but in recent days the militants have displayed the tactics to which they are likely to resort if they lose the city, killing dozens with bombs in Baghdad and attacking security forces elsewhere.