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 – borne not of drought or floods but a vicious conflict that destroyed the livelihoods of farmers like Yaw and then prevented aid workers from entering their villages.

A U.N. declaration of famine in February was supposed to bring a surge of assistance to this northern county.

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.

GROWING PAINS

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 believes residents support the rebels. The U.S. deputy ambassador to the United Nations, Michelle Sisson, said last week 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 entire cities. Humanitarian groups were recently stunned to learn that the government was considering requiring a $10,000 license for every foreign aid worker in the country.

ROGUE SOLDIERS BLAMED

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.

“Individual officers might stop a humanitarian convoy and harass humanitarian workers, but that doesn’t represent the view of the government,” said Hussein Mar, the minister of humanitarian affairs. “In a war situation, there are people who will take the law into their own hands.”

South Sudanese leaders on both sides of the conflict rarely acknowledge the impact of their restrictions on aid workers.

“It is extraordinary in a place where a famine has been declared for the first time in five years that we’re not hearing more from the leadership about the problems facing the people,” said David Shearer, the top U.N. official in South Sudan, in an interview.

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. The findings for 2016 have not yet been released. Seventy-nine aid workers have been killed since the war began, including six who were murdered last Saturday in an ambush on the road from Juba to Pibor, in the east.

NOBODY’S BLAMELESS

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.

Recently, on a 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 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 (WFP) country director, who was on the trip.

Mayendit’s descent into famine took years, as spurts of violence ravaged the county, eroding the ability of farmers and herders to provide for themselves.

Aid officials warned again and again that the county was falling apart. Without a political solution to the war, they said, they would be racing to keep people alive after each clash. That political solution never came.