WFP called on Sudan's fighting factions, the Sudanese Armed Forces, SAF, and the paramilitary Rapid Support Forces, RSF, to provide immediate guarantees for the unimpeded delivery of relief.
Nearly 18 million people across Sudan are facing acute hunger, and more than five million are experiencing emergency levels of hunger in areas worst affected by the conflict.
WFP said in a statement that it was able to deliver aid to just one in 10 people in those areas, which include the capital Khartoum, the western region of Darfur, and El Gezira state, where the RSF recently advanced.
"It is becoming nearly impossible for aid agencies to cross due to security threats, enforced roadblocks, and demands for fees and taxation," the WFP statement said.
Fighting between SAF and the RSF began in April 2023 when a power struggle between the army and the paramilitary group erupted over a plan to shift towards civilian rule.
The two sides shared power with civilians after the overthrow of former leader Omar al-Bashir in 2019, before derailing that transition by staging a coup together in 2021.
Efforts to negotiate an end to the fighting have so far yielded no breakthrough.
The WFP said it was trying to obtain security guarantees to restart operations in El Gezira, previously an aid hub that many had fled to from Khartoum.
Aid deliveries in Sudan had been limited because 70 trucks had been stuck in Port Sudan for more than two weeks, and another 31 had been stuck in El Obeid for more than three months, the agency said. Both cities are controlled by the army.
"Every single one of our trucks need to be on the road each and every day delivering food to the Sudanese people," said Eddie Rowe, WFP representative in Sudan.
"Yet life-saving assistance is not reaching those who need it the most, and we are already receiving reports of people dying of starvation."
Under a classification agreed by a partnership of U.N. agencies and NGOs, crisis levels of hunger mean households are suffering from high rates of acute malnutrition or can only meet minimum needs through crisis-coping strategies or using up essential assets.
Emergency levels of hunger means households are suffering from very high acute malnutrition or death or can only cope through emergency measures or liquidating assets.