United Nations secretary-general Antonio Guterres has said the UN needs £3.5 billion by the end of next month to prevent "a catastrophe" of hunger and famine in South Sudan, Nigeria, Somalia and Yemen.
Just £72 million has been collected so far.
Mr Guterres said more than 20 million people altogether are going hungry in those countries.
Amplifying concerns that other UN officials expressed recently, Mr Guterres said the world "is facing a tragedy" and "must avoid it becoming a catastrophe".
Children’s agency Unicef warned on Tuesday that almost 1.4 million children are at "imminent risk of death" from acute malnutrition this year in the four countries.
Famine was declared on Monday in parts of South Sudan’s Unity state.
Mr Guterres said he believes governments and other donors will step up financially to help.
His comments come just hours after Pope Francis demanded concrete action to get food aid to famine victims in South Sudan, saying words are not enough to prevent millions from being condemned to death by hunger.
His appeal came a day after South Sudan President Salva Kiir promised "unimpeded access" for all aid organisations to reach the hungry.
South Sudan has repeatedly promised such access but with little effect.
Pope Francis said: "At this time it’s more necessary than ever for everyone to not just stop with words, but to take concrete action so that food aid can reach suffering populations."
The UN earlier this week declared a famine in parts of oil-rich Unity state, saying more than 100,000 people are affected and that one million more are on the brink of starvation.
Emmanuel, age 2, waiting for #milk to help ensure he recovers from #malnutrition. In a POC site in #Juba, #SouthSudan. RT to show support. pic.twitter.com/DwRcHN3LrN— UNICEF South Sudan (@unicefssudan) February 21, 2017
Reporting: PA and AP