The World Food Programme has demanded Yemen's rebel Houthi movement stops diverting desperately needed food aid from people in areas under its control.
A survey by the UN agency said people in the capital Sanaa had not received rations to which they were entitled.
The WFP said lorries were illegally removing food from distribution areas, with rations sold on the open market or given to those not entitled to it.
There was no response from the Houthis, but they have denied diverting aid.
The UN says some 20 million Yemenis are food insecure and that 10 million of them do not know how they will obtain their next meal.
Yemen has been devastated by a conflict that escalated in 2015, when a Saudi-led coalition intervened after the Houthis seized control of much of the west of the country and forced President Abdrabbuh Mansour Hadi to flee abroad.
At least 6,800 civilians have been killed and 10,700 injured in the fighting, according to the UN. Thousands more civilians have died from preventable causes, including malnutrition, disease and poor health.
The WFP said the misappropriation of aid came to light in a review conducted in recent months, prompted by an increasing number of reports of humanitarian food for sale on the open market in Sanaa.
Checks unearthed fraud being perpetrated by at least one local partner organisation tasked with handling and distributing food assistance, according to the agency. The local organisation is affiliated with the Houthi-run education ministry.
"This conduct amounts to the stealing of food from the mouths of hungry people," said WFP Executive Director David Beasley in a statement.
"At a time when children are dying in Yemen because they haven't enough food to eat, that is an outrage. This criminal behaviour must stop immediately."
The WFP said its monitors had amassed photographic and other evidence of lorries illicitly removing food from designated food distribution centres.
They also found that the selection of beneficiaries was being manipulated by local officials and that food distribution records were being falsified.
Some food relief was being given to people not entitled to it and some was being sold for gain at markets in the capital, according to the WFP.
Mr Beasley warned the Houthi authorities in Sanaa that unless they took immediate action to end the diversion of aid the WFP would "have no option but to cease working with those who have been conspiring to deprive large numbers of vulnerable people of the food on which they depend".
On 13 December, the rebels and Yemen's government agreed to a UN-brokered ceasefire in the key Red Sea port and city of Hudaydah, which is crucial for the delivery of aid supplies.
On Saturday, the Houthis said it had withdrawn its fighters from the port and handed over control to the coast guard. But UN spokesman Stephane Dujarric questioned the claim, saying such steps could only be credible if all other parties could observe and verify them.
Mr Dujarric also said the Houthis had failed to honour an agreement to open a humanitarian corridor along the Hudaydah-Sanaa highway by Sunday as agreed.