The World Food Programme plans to suspend some of its work in Yemen within days in protest against repeated thefts of aid by Houthi rebels, which are frustrating the work of UN staff.
The halting of food distribution in the capital Sanaa could be expanded if the stealing continues, David Beasley, the programme's executive director, said in New York.
On Monday, Mr Beasley told the UN Security Council that the Houthi leadership failed to address the problem over the past 18 months.
“Let me be crystal clear: children are dying right now because of this,” he said.
“There are those who stand to make a profit and they will do everything to obstruct and delay.”
The agency is trying to feed about 12 million of Yemen's most vulnerable – nearly half of the population – at a cost to the international community of about US$150 million (Dh550.9m) a month.
Many areas are on the brink of being declared famine zones and the UN has called the situation the worst humanitarian disaster in the world.
Agreements signed in January between the UN and authorities in Sanaa have either been ignored or subverted.
“Aid is being diverted for profit,” Mr Beasley said, giving 30 cases in the past two months where the work of UN staff was stopped and food was not passed to civilians.
“Our food assistance is being manipulated. Assistance is going to the wrong people."
Mr Beasley said that unless the situation improved immediately, the agency would stop its work.
“If we do not receive these assurances, then we will begin a phased suspension of food assistance, most likely towards the end of the week,” he told the council.
“If not? Then we are looking at expanding the suspension to other areas where the risk of diversion is high. We don't want to do this. It goes against the fibre of our being.”
A nutrition programme for malnourished children, pregnant women and new mothers will be unaffected.
Mr Beasley's announcement was accompanied by warnings from other UN officials about Yemen.
Mark Lowcock, UN undersecretary general for humanitarian affairs, compared the situation to Groundhog Day, a film where the same events recur day after day.
While a limited ceasefire in the Red Sea port of Hodeidah has reduced violence there, conflict has intensified elsewhere in the country, Mr Lowcock said.
Martin Griffiths, the UN special envoy to Yemen, told the council there were still problems in implementing the Hodeidah ceasefire.