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

The threat of banditry on Darfur's roads - by both rebel factions and government militias - during the last year has forced aid groups to increasingly rely on helicopters and other flights to gain access to the region, where 2.5 million people are displaced because of conflict.

The air transport is provided by the United Nations' Humanitarian Air Service. But funding for the service, which costs $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 for lack of funds.

The United Nations said the air service - a fleet of 20 planes and six helicopters - needed $20 million by Sunday 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 Laurent Bukera, head of the North Darfur Area Office for the World Food Program, referring to donors. "After so many years, people think that this service is a given."

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

The United Nations estimates as many as 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 banditry to support their causes.

Their main target: aid groups, whose Toyota Land Cruisers and trucks are routinely hijacked along Darfur's roads, sold for cash, or modified into battle wagons.