Like others interviewed, Abu Marwan, citing concerns about safety, agreed to speak on the condition that his full name not be published. He and others are identified by honorifics or nicknames. Residents said anyone caught using a cellphone risked being beaten or killed by militants.
Abu Salah, who lives in the Bab al-Tob neighborhood in western Mosul, said the Islamic State, also known as ISIS, had recruited three families as spies in return for food. He said they had identified eight young men who were subsequently arrested by militants and accused of providing information to Iraqi security forces.
“We call this food for information,” Abu Saleh said. “These are poor families. Hunger pushes them to abandon their principles.”
The United Nations in Iraq said on Saturday that most of the 750,000 to 800,000 residents of western Mosul had been living under extreme duress for weeks, creating a humanitarian crisis even before the start of military operations.
Once the battle for western Mosul begins, a top United Nations official said, 250,000 to 400,000 people may try to flee. Emergency sites are being built south of Mosul, where food and other supplies are being gathered to accommodate a rush of displaced people.
“Tens of thousands of people may flee or be forced to leave the city,” Lise Grande, the United Nations’ humanitarian coordinator for Iraq, said in a statement on Saturday. “Hundreds of thousands of civilians might be trapped — maybe for weeks, maybe for months.”
Ms. Grande added: “People, right now, are in trouble. We are hearing reports of parents struggling to feed their children and to heat their homes.”
Few, if any, commercial supplies have reached western Mosul in the three months since the main road to Syria was cut, the United Nations said recently. With cooking gas and kerosene scarce, families are burning wood, plastic, furniture and garbage to heat their homes and cook their food.
The United Nations said that most essential food items were virtually unattainable. Half of all food shops have closed, and others have little for sale. Staples like rice, flour, legumes, date syrup, baby formula and sugar have almost disappeared. There is enough cooking oil to supply only 10 percent of the population.
Umm Aisha, a widow with three children, said her two sons, ages 7 and 5, were weak and listless from hunger. She said her 10-year-old daughter’s face had turned yellow and sunken. “I can’t provide enough food to keep them alive,” she said, weeping.
Food prices in western Mosul are almost twice as high as those in eastern Mosul, Sally Haydock, the representative of the World Food Program in Iraq, said in a statement. “We are extremely concerned that many families do not have enough to eat,” she said.
The price of a bag of flour has skyrocketed to 150,000 Iraqi dinars, or about $130, said a 41-year-old resident of Bab al-Tob.
Just 40 percent of residents have access to safe drinking water, with others using untreated water from private wells, said Peter Hawkins, Unicef’s representative in Iraq. Most homes receive only two to three hours of electricity a day, and militants have recently reduced even those levels.
American military officials said on Saturday that while the Mosul offensive was an Iraqi-led fight, American firepower and expertise would support it.
Some of the 450 American advisers in Iraq will be helping Iraqi officers plan and execute the offensive. American and allied airstrikes in support of the Iraqi forces are expected to increase.
American commanders say about 2,000 to 3,000 Islamic State fighters remain in western Mosul after the liberation of the city’s eastern half last month.
“Some are hard core and will stand and fight and die in place, some will want to escape, some will attempt to quietly quit the fight, and some will sympathize with the enemy but are not now participating,” Col. John Dorrian, a military spokesman in Baghdad, said in an email. “And then, unfortunately, some will be pressed into the fight under threat of death or other harsh retribution.”