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 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.