Tens of thousands of people trapped in remote areas of South Sudan have received food aid through airdrops, the United Nations World Food Programme (WFP) said on Monday, as growing conflict in the region threatens to push entire communities into famine.

The agency said the deliveries, carried out by air, marked the first time in more than four months it had been able to reach people in the isolated counties of Nasir and Ulang.

"These distributions mark WFP's first access in over four months to deliver life-saving food and nutrition assistance to more than 40,000 people... in the most remote parts of Nasir and Ulang counties, areas only accessible by air," the agency said in a statement.

The aid comes as violence surges in parts of South Sudan following renewed tensions between President Salva Kiir and Vice President Riek Machar, which escalated into open conflict in March.

The outbreak has sparked fears of a return to full-blown war in a country still recovering from a brutal civil war that lasted five years and killed around 400,000 people.

The fighting has had a devastating effect on the humanitarian situation in Upper Nile state, which borders Sudan. According to the WFP, more than one million people in the region are facing acute food insecurity.

Among them, over 32,000 people are already experiencing "catastrophic" levels of hunger — a number that has tripled since the violence escalated and displaced large numbers of families.

The WFP also said the conflict has made it impossible to use the main river routes that typically serve as the cheapest way to move supplies to Upper Nile and northern Jonglei. This has forced the agency to turn to expensive airdrops to reach isolated populations.

Across the country, 7.7 million people about 57 percent of the population, are currently experiencing either crisis, emergency or catastrophic levels of hunger, the agency warned.

A critical lack of funding has made matters worse, forcing the WFP to reduce both the quantity and reach of its assistance. Currently, only the most vulnerable 2.5 million people are receiving help, and even then, the aid is being delivered in reduced rations.

The agency has appealed for $274 million (Sh35.3 billion) to keep its operations running through December and continue supporting South Sudanese communities caught in the middle of deepening conflict and a worsening hunger crisis.