GENEVA — South Sudan needs $230 million in international aid in the next 60 days or it will face the worst starvation in Africa since the 1980s, when hundreds of thousands of people died in Ethiopia’s famine, the United Nations official coordinating humanitarian aid in South Sudan warned on Thursday.

“We’re in a race against time,” the coordinator, Toby Lanzer, told reporters in Geneva. In a stark message to world leaders, he said, “Invest now or pay later.”

About 3.7 million people, close to one-third of the total population, are already at severe risk of starvation in South Sudan, a crisis now ranked by the United Nations on par with Syria’s, Mr. Lanzer said. He appealed for only the most essential needs, food, water, seeds and farming tools, to allow the South Sudanese to plant crops before the end of May, when rains bring the planting season to an end.

“If we miss the planting season, there will be a catastrophic decline in food security,” Mr. Lanzer said. “What will strike that country, and it will hit about seven million people, will be more grave than anything that continent has seen since the mid-1980s.”