The UN World Food Programme is cutting air services to Darfur, reducing the ability of 14,000 aid workers to travel to the Sudanese region.
The agency said it was cutting one of six helicopters with immediate effect, and two aeroplanes on 19 June.
The WFP said it needed $20m (£10m) by 15 June to avoid some of the cuts and maintain a full service.
Meanwhile, Sudan's government has said it wants global police agency Interpol to arrest 20 Darfuri rebel leaders.
The men are wanted for their alleged role in an attack near the capital, Khartoum, last month.
The raid at Omduran was the closest rebel groups had come to Khartoum in five years of conflict in which an estimated 300,000 people have died and two million have fled their homes.
Justice Minister Abd-al-Basit Sabdarat said the wanted men included the leader of the Justice and Equality Movement rebel group, Khalil Ibrahim.
A spokesman for the group said its leaders were not worried because, he said, they had not committed any crime.
Higher fees
The BBC's Amber Henshaw in Khartoum says the reductions will be a huge blow to aid agencies in the region.
About 3,000 humanitarian workers use the helicopter service run by the WFP.
Travel by road to remote parts of Darfur is impossible because of insecurity, banditry or poor road conditions.
The WFP said it would also be increasing fees for helicopter travel from $40 to $100 per flight on 1 July.
Kenro Oshidari, the head of the WFP in Sudan, said the agency had been facing the possible closure of air services since March because of a lack of funding.
The cuts were aimed at keeping vital operations going for longer while it waited for extra funding, he added.
A spokesman for Oxfam in Khartoum said the changes would have a big impact on their ability to reach remote areas in Darfur.
He said increased insecurity in the region meant they relied on the service to get to four out of their seven projects and that any further cuts could be disastrous.
Since January the UN-African Union force has not been able to bring peace to Darfur. It has just 9,000 of the planned 26,000 troops.
The violence in Darfur began in 2003 when rebel groups complaining of discrimination against black Africans began attacking government targets.
The government mobilised what it called "self-defence militias" in response, but denies any links to the Janjaweed, accused of trying to "cleanse" black Africans from Darfur.
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