NAIROBI, Kenya — Humanitarian flights that deliver doctors, aid workers, and supplies to remote areas of Sudan’s western Darfur region are being cut because of lack of funding, the U.N. World Food Program said yesterday.

The threat of bandits on Darfur’s roads during the past year has forced aid groups to increasingly rely on helicopters and other flights to gain access to the region, where an estimated 2.5 million people are displaced because of conflict.

The air transport is provided by the U.N.’s Humanitarian Air Service. But funding for the service, which costs about $77 million a year, has become tenuous as the conflict has dragged into its fifth year.

Yesterday, the air service grounded one of its six helicopters and reduced the number of flights because of lack of funds.

The United Nations said the air service — a fleet of 20 planes and six helicopters — needs an infusion of $20 million by June 15 to maintain full service in the next few months. The program has a total budget shortfall of $48 million this year, U.N. officials said.

“People are weary,” said the head of the North Darfur Area Office for the World Food Program, Laurent Bukera, referring to donors. “After so many years, people think that this service is a given. I find this really worrying that we have to wait until the last minute” to scramble for funds.

About 14,000 aid workers are in Darfur, which is home to the largest humanitarian relief effort in the world. Ms. Bukera said that the loss of one helicopter immediately translates into hundreds of aid workers being grounded.

Experts estimate that 400,000 people have died during the conflict in Darfur, which began with the Sudanese government and its allied militias waging a brutal campaign to crush rebels who complained of economic and social injustice. But the nature of the insecurity in Darfur has changed since 2003, with the fractious rebels and government militias turning to carjacking and other banditry to support their causes.