"To be precise, we need $4.4 billion by July", Mr O'Brien told the UN Security Council on Friday.
Fears are rife "a catastrophe" could rock the global with more than 20 million people faced with starvation and famine in Yemen, Somalia, South Sudan and Nigeria.
O'Brien, speaking to the United Nations security council in NY, issued a plea for $4.4 billion by July to "avert a catastrophe" in the countries.
The world is facing the worst humanitarian crisis since 1945 as more than 20 million people in four counties will starve to death, the United Nation (UN) humanitarian chief says.
The UN Secretary General also stated that 1.4 million children are susceptible to imminent death due to starvation in the four countries, while 462,000 children face acute malnutrition in Yemen alone due to the civil war that has been raging for two years.
"Without collective and coordinated global efforts, people will simply starve to death. Millions of lives are at risk and more will die if we do not act quickly and decisively", the World Bank boss noted.
Without a major infusion of money, he said, children will be stunted by severe malnutrition and will not be able to go to school, gains in economic development will be reversed and "livelihoods, futures and hope will be lost". Communities' resilience is rapidly wilting away.
If the situation doesn't change, he warned that many people "will be displaced and will continue to move in search for survival, creating ever more instability across entire regions".
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Each country listed above suffers from very different circumstances.
In total, more than 7.5 million people there need aid.
The conflict, which has left around 20,000 people dead and forced more than 2.6 million others to flee their homes, has aggravated an already hard humanitarian situation in one of the poorest regions of the world.
The drought-related crisis of food is worsened by the increased number of displaced people due to the wars in Sudan, South Sudan and Somalia, where 50,000 people are seeking refuge coming from Mogadishu, Afgooye, Baidoa, Kismayo, Dolow and Beledweyne.
"That is three million people more than in January", he said.
In the last starvation in Somalia, about six years ago, almost 260,000 people died.
Fighting on the ground and air strikes on areas held by Houthi rebels, targeted by a Saudi-led coalition backed by the USA and United Kingdom, have displaced more than three million people. "We're ready despite incredible risk and danger... but we need those huge funds now".
UN Secretary General Antonio Guterres called last month for more money, saying only $90 million had been received so far - about two cents for every dollar of the $5.6 billion needed for aid this year.