Video report by ITV News Correspondent Martha Fairlie
The world faces its largest humanitarian crisis since 1945 with over 20 million people in four countries facing starvation and famine, the United Nations has said.
UN humanitarian chief Stephen O'Brien called for an immediate injection of funds for South Sudan, Somalia, Yemen and Nigeria.
"To be precise, we need $4.4 billion by July," Mr O'Brien told the UN 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
children are at "imminent risk of death" due to famine across the four countries, Unicef has said
Two-thirds of Yemenis 'need aid'
Mr O'Brien said the largest humanitarian crisis is in Yemen where two-thirds of the population - 18.8 million people - need aid.
"More than seven million don't know where their next meal will come from,"
"That is three million more chronically hungry people than in January," he added.
South Sudan situation "worse than it has ever been"
In South Sudan, the world's newest country, the situation "is worse than it has ever been," the humanitarian chief said.
The world's newest country, which has been blighted by a three-year civil war and economic turmoil, is in the grip of the world's first famine for six years.
The UN has warned 100,000 people face starvation with a further 1 million at risk.
According to Unicef, more than 270,000 children are suffering severe malnutrition in South Sudan.
ITV News Africa correspondent John Ray recently visited the country and witnessed first-hand the human cost of the famine.
Malnutrition rife in northeast Nigeria
In northeast Nigeria, more than 20,000 people have been killed during a seven-year uprising by the Boko Haram Islamic extremist group.
The conflict has also driven an estimated 2.6 million from their homes.
In February, a UN humanitarian coordinator said malnutrition in the northeast is so pronounced that some communities many young children have died.
Somalia at 'incredible risk' of famine
Meanwhile, in Somalia, which Mr O'Brien also visited, more than half the population 6.2 million people are said to need humanitarian assistance and protection
This includes 2.9 million who are at risk of famine and require immediate help "to save or sustain their lives," he said.
Mr O'Brien also warned that close to one million children under the age of five will be "acutely malnourished" this year.
"To be clear, we can avert a famine," O'Brien said.
"We're ready despite incredible risk and danger ... but we need those huge funds now."
ITV News International Affairs Editor Rageh Omaar has visited Somaliland - which gained independence from the rest of the country in 1991 - where a severe drought has brought it to crisis.