UN humanitarian chief Stephen O'Brien said that more than 20 million people faced the threat of starvation and famine in Yemen, Somalia, South Sudan and Nigeria.
Unicef has already warned 1.4m children could starve to death this year.
Mr O'Brien said $4.4bn (£3.6bn) was needed by July to avert disaster.
"We stand at a critical point in history," Mr O'Brien told the Security Council on Friday. "Already at the beginning of the year we are facing the largest humanitarian crisis since the creation of the United Nations."