MAYENDIT, South Sudan — When his stepbrother starved to death in January, Matthew Yaw buried him in the sand next to the family’s shack of sticks and plastic, one more grave at the epicenter of the world’s most severe hunger crisis.

It is a man-made disaster — born not of drought or floods but a vicious conflict that destroyed the livelihoods of farmers like Yaw and then prevented aid workers from their villages.

A U.N. declaration of famine in February was supposed to bring a surge of assistance to the nascent nation. But within days, the South Sudanese government ordered aid workers to leave ahead of a planned offensive, and the area was soon consumed with fighting.

Yaw and his neighbors have been reduced to eating water lilies and an occasional fish from a nearby river. The few relief workers who managed to visit Mayendit County in recent days saw people languishing half-naked. Their clothes had been burned in the last attack.

There are now four hunger crises across the Middle East and Africa in what is emerging as the greatest humanitarian disaster since World War II, according to the United Nations.

In each place — Nigeria, Somalia, Yemen and South Sudan — aid workers are being blocked from reaching the needy, in some cases by insurgents, in others by soldiers or bureaucratic restrictions. Twenty million people across the four countries could starve if they don’t quickly get help, according to the United Nations.

“When you get one month of food for three months, you go hungry,” said Yaw, 37, a tall man who leaned on a wooden reed, his ankle shattered last year by a bullet as he fled the fighting.

Five years ago, the world celebrated South Sudan’s emergence as the world’s newest country, following a peace process with Sudan that was championed by Washington.

But in 2013, a clash broke out between the nation’s president and vice president, soon becoming a broader ethnic conflict. As many as 50,000 people have been killed. More than 40 percent of South Sudan’s 12 million people are now classified as “food insecure.”

The warring parties — particularly government troops — have restricted humanitarian assistance in ways large and small.

Some of their actions appear to be brute thuggery, like the theft by soldiers last summer of more than 4,000 tons of food from a warehouse in Juba, the capital, enough to feed 220,000 people for a month.

But aid workers fear the government is intentionally denying aid to regions where it thinks residents support the rebels. The U.S. deputy ambassador to the United Nations, Michelle Sisson, said recently that the government’s actions “may amount to deliberate starvation tactics.”

There are now more than 70 checkpoints on the 400-mile stretch of road between the capital and Bentiu, a major city north of Mayendit, with soldiers and other armed men demanding money or food before allowing aid trucks to continue.

At least 80 times a month, according to a U.N. tally, the South Sudanese authorities and rebels reject permits for planes to take off bearing emergency food or medical aid, or deny access to cities.

South Sudanese officials say that the government doesn’t have a policy of obstructing aid, but that the country’s dire economic situation has led to rogue soldiers making their own demands.

Aid workers are often caught in the crossfire. In 2015, there were 31 attacks against relief workers in South Sudan, more than any other country in the world, according to the Aid Worker Security Database maintained by the research group Humanitarian Outcomes.

Seventy-nine aid workers have been killed since the war began, including six who were murdered recently in an ambush on the road from Juba to Pibor, in the east.

In Mayendit, one of two regions officially experiencing famine, the greatest barrier to reaching starving residents has been the near-constant fighting between government forces and rebels. In some cases, even after the United Nations airdropped food, soldiers ransacked villages and stole the provisions from civilians.

On a recent scorching afternoon, a small team of U.N. officials landed in Mayendit in a white helicopter, trying to figure out what they could do to improve their access to the hungry. It was a particularly tense moment. Eight aid workers from the North Carolina-based charity Samaritan’s Purse had recently been detained in the area for a day by rebels. There were rumors that government forces were planning another attack.

“They can’t behave like this and expect humanitarians to continue going in,” said Joyce Luma, the World Food Program country director, who was on the trip.

The U.N. team disappeared into a run-down building with rebel leaders. They had become accustomed to this kind of negotiation; nearly every food drop, convoy and official visit requiring a litany of permits and diplomatic entreaties. A WFP team now keeps a satellite phone with numbers for rebel and government commanders.

In some cases, relief workers have been able to persuade commanders to delay offensives while they deliver bags of food. But in many others, they have not.

In Juba, aid officials said privately that the government was restricting assistance to starve those it perceived as its enemies, including women and children in rebel-held regions like Mayendit.

But the aid officials, fearing that their efforts will be further impeded, have been reluctant to speak publicly about such tactics.

“When the government carries out a counterinsurgency campaign, they end up treating civilians as the enemy,” said one senior relief official.

The lack of food isn’t the only problem — cholera has broken out because of the scarcity of clean water and poor sanitation.

The only hospital in the region, in Leer, was looted four times in two years, with medicine, equipment and fuel stolen. Doctors Without Borders, the global medical charity, closed the hospital earlier this year and instead dispatched small, lesser-resourced health teams to Mayendit.

Thousands have poured out of the county, walking for days to reach displacement camps like one in Ganyiel.

“We came here because we were tired of our food being stolen,” said James Gawar, 35. “Our children were sick. We needed a place where there was help.”