The United Nations said on Tuesday there were indications that poorer families in Mosul are struggling to feed themselves as food prices increase following the U.S.-backed offensive on the IS group-held city in northern Iraq.

"Key informants are telling us that poor families are struggling to put sufficient food on their tables," U.N Humanitarian Coordinator in Iraq, Lise Grande, told Reuters. "This is very worrying."

Iraqi government and Kurdish forces surround the city from the north, east and south, while Popular Mobilisation forces - a coalition of Iranian-backed Shia groups - are trying to close in from the west. Last week the Popular Mobilisation forces cut off the supply route to Mosul from Islamic State-held territory in Syria.

"In a worst case, we envision that families who are already in trouble in Mosul will find themselves in even more acute need." Grande said. "The longer it takes to liberate Mosul, the harder conditions become for families."

Six weeks after the launch of the offensive, Iraqi forces moving from the east have captured about a quarter of the city, trying to advance to the Tigris river that runs through its centre. A U.S.-led coalition is providing air and ground support to the offensive that started on Oct. 17.