"People will start dying of starvation. They have no doctors and no food. There is no flour, no bulgur wheat, no rice, no milk, there is nothing to eat."
The shortages have sent food prices soaring and in the few places where markets are still open many families cannot afford to buy anything. There have been reports of people eating cats to ward off hunger.
"What we're hearing from inside western Mosul right now is deeply concerning," said Maurizio Crivallero, Save the Children's director in Iraq.
"Even when there is food available at the markets, people don't have any money left to buy it. Families and their children are on the brink of starvation."
Some residents compared the situation to the Syrian city of Madaya, where people in the rebel-held town starved to death while under siege from the Syrian regime.
Unlike in previous battles in Ramadi and Fallujah, the vast majority of Mosul residents have stayed in their homes as Iraqi forces close in rather than fleeing.
The continuing civilian presence has preserved some sense of normality in the city but also made it harder for humanitarian agencies to reach people who require aid. More than half of those 750,000 people in the west of the city are younger than 18, according to Mercy Corps.
Isis fighters have reportedly executed people caught trying to escape from their areas of control. About 3000 Isis troops are believed to still be inside Mosul although most of the senior leaders have fled to Syria.
Fighting is focused around the remains of Mosul International Airport, which Iraqi forces hope to seize before starting their main drive into the remaining Isis areas.
The airport was opened to civilian traffic in 2007 after years of disuse under Saddam Hussein and during the Iraq war but shut when Isis seized control of the city in 2014.
The jihadists appear to have destroyed much of the airport during their two-year reign, digging deep trenches across its two runways in an apparent effort to sabotage them.
The airport and a nearby military base will be used as staging bases for the thrust into western Mosul, the Iraqi military said.
The western half of the city may be more difficult to clear than the east because its ancient districts contain a tangle of alleyways that are too narrow for tanks and armoured vehicles.
Airwars, a group that monitors airstrikes in Iraq and Syria, said it had seen a sharp rise in civilian casualties in Mosul as coalition airstrikes pounded the city.
Around 170 civilians were likely killed by coalition air strikes in Mosul last month, the group said, compared with about 80 civilian casualties in December.
In one incident on January 3 as many as 22 civilians were reportedly killed by a single coalition strike in eastern Mosul. One family claimed to have lost 11 members in the strike.
