The World Food Programme has paused food deliveries to northern Gaza, saying aid convoys had endured “complete chaos and violence due to the collapse of civil order”.
The agency says the decision has not been taken lightly and crews had faced crowds, gunfire and looting.
The UN has been warning of looming famine in the north since December.
The WFP says these latest reports are proof of a “precipitous slide into hunger and disease”.
The Israeli military ordered 1.1 million Palestinian civilians to evacuate all areas north of Wadi Gaza and seek shelter in the south at the start of its ground offensive in October.
The evacuation area included Gaza City – which before the war was the most densely populated area of the territory.
Most residents followed the Israeli order, but several hundred thousand chose to stay or were unable to flee as Israeli troops encircled the region and then largely took control of Hamas strongholds there.
Last month the UN agency for Palestinian refugees, Unrwa, said at least 300,000 people who had remained in northern Gaza depended on its assistance for their survival.
Aid deliveries to the north have been scarce and dependent on security clearances from the Israeli military.
This weekend the WFP had hoped to begin a week-long delivery, sending 10 lorries each day to help “stem the tide of hunger and desperation”.
“Food and safe water have become incredibly scarce and diseases are rife, compromising women and children’s nutrition and immunity and resulting in a surge of acute malnutrition,” it said.
