As Sudan hurtles toward famine, its military is blocking the United Nations from bringing enormous amounts of food into the country through a vital border crossing, effectively cutting off aid to hundreds of thousands of starving people during the depths of a civil war.

Experts warn that Sudan, barely functioning after 15 months of fighting, could soon face one of the world’s worst famines in decades. But the Sudanese military’s refusal to let U.N. aid convoys through the crossing is thwarting the kind of all-out relief effort that aid groups say is needed to prevent hundreds of thousands of deaths — as many as 2.5 million, according to one estimate — by the end of this year.

The risk is greatest in Darfur, the Spain-sized region that suffered a genocide two decades ago. Of the 14 Sudanese districts at immediate risk of famine, eight are in Darfur, right across the border that the United Nations is trying to cross. Time is running out to help them.

The closed border point, a subject of increasingly urgent appeals from American officials, is at Adré, the main crossing from Chad into Sudan. At the border, little more than a concrete bollard in a dried-out riverbed, just about everything seems to flow: refugees and traders, four-wheeled motorbikes carrying animal skins, and donkey carts laden with barrels of fuel.

What is forbidden from crossing into Sudan, however, are the U.N. trucks filled with food that are urgently needed in Darfur, where experts say that 440,000 people are already on the brink of starvation. Refugees fleeing Darfur now say that hunger, not conflict, is the main reason they left.