UNITED NATIONS — The world faces the largest humanitarian crisis since the United Nations was founded in 1945 with more than 20 million people in four countries facing starvation and famine, the U.N. humanitarian chief said Friday.

Stephen O’Brien told the U.N. Security Council that “without collective and coordinated global efforts, people will simply starve to death” and “many more will suffer and die from disease.”

He urged an immediate injection of funds for Yemen, South Sudan, Somalia and northeast Nigeria plus safe and unimpeded access for humanitarian aid “to avert a catastrophe.”

“To be precise,” O’Brien said, “we need $4.4 billion by July.”

Without a major infusion of money, he said, children will be stunted by severe malnutrition and won’t be able to go to school, gains in economic development will be reversed and “livelihoods, futures and hope will be lost.”

U.N. and food organizations define famine as when more than 30 percent of children under age 5 suffer from acute malnutrition and mortality rates are two or more deaths per 10,000 people every day, among other criteria.

“Already at the beginning of the year we are facing the largest humanitarian crisis since the creation of the United Nations,” O’Brien said. “Now, more than 20 million people across four countries face starvation and famine.”

O’Brien said the largest humanitarian crisis is in Yemen, where two-thirds of the population – 18.8 million people – need aid and more than 7 million people are hungry and don’t know where their next meal will come from. “That is 3 million people more than in January,” he said.

The Arab world’s poorest nation is engulfed in conflict and O’Brien said more than 48,000 people fled fighting just in the past two months.

O’Brien said $2.1 billion is needed to reach 12 million Yemenis with life-saving aid.