The WFP has suspended some of its aid deliveries
The UN food agency says it is facing critical funding shortages that have forced it to cut aid deliveries to millions of people facing starvation.
The World Food Programme (WFP) said it could have to close parts of its airway, used to fly aid workers to humanitarian trouble-spots.
Deliveries have already been suspended to north Uganda, Ivory Coast and Niger.
The organisation has issued similar warnings in the past when facing funding shortages.
The UN Humanitarian Air Service (UNHAS), operated by WFP, has a budget for 2009 of $160m (£96m) but has received less than $90m in fees and contributions this year.
WFP spokesman Greg Barrow said UNHAS was "a vital component of humanitarian operations across the world".
"But because of a funding shortfall there is now a grave risk that the air service ... could literally be grounded in the next few weeks due to a lack of funds," he said.
WFP said funding for the airline's Chad service will run out on 15 August and needs $6.7m (£4m) to continue flying to the end of the year.
Spokeswoman Emilia Casella said the single-plane service flies an average of 4,000 humanitarian passengers to and from Chad each month.
She said the cancellation would not stop food deliveries taking place, but would mean that aid workers would not be able to reach communities that need them most.
The service supplying Liberia, Sierra Leone and Guinea needs $3.3m (£1.9m) to continue flying to the end of the year.
Pierre Carrasse, Chief of WFP's Aviation Branch, asked how workers could reach the often remote areas affected by conflict without the airline.
"How will WFP reach the hungry? How will doctors reach their patients? How will people have clean water if the engineers who help to build wells can't get there," he asked.
Shortages have already led to UNHAS closing its service in Ivory Coast in February.
The Niger service, also suspended that month, is expected to resume in August after a recent donation from the UN Common Emergency Relief Fund.
The UN says 102 million people in 78 countries received food aid last year.