The world faces the largest humanitarian crisis since the United Nations was founded in 1945, the UN's humanitarian chief has warned.

Stephen O'Brien told the UN Security Council that over 20 million people in Yemen, South Sudan, Somalia and northeast Nigeria are facing starvation and famine.

He said 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 the four countries plus safe and unimpeded access for humanitarian aid "to avert a catastrophe".

"To be precise, we need $4.4bn (£3.6bn) by July," said Mr O'Brien.


Famine declared in South Sudan

Without the money, he said, children will be stunted by severe malnutrition, gains in economic development will be reversed and "livelihoods, futures and hope will be lost".

UN and food organisations define famine as when more than 30% of children under the age of five suffer from acute malnutrition and mortality rates are two or more deaths per 10,000 people every day, among other criteria.

Mr 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 seven million people are hungry.

Image: A girl searches a rubbish dump for pieces of food in the port city of Hodeidah in Yemen

The Arab world's poorest nation is engulfed in conflict and the UN says more than 48,000 people have fled fighting just in the past two months.

South Sudan, the world's newest nation, has been ravaged by a three-year civil war and more then 7.5 million people are in need aid.

"The famine in South Sudan is man-made," said Mr O'Brien. "Parties to the conflict are parties to the famine - as are those not intervening to make the violence stop."

Suffering and starvation: Growing up in Yemen

In Somalia more than half the population - 6.2 million people - need humanitarian assistance, including 2.9 million who are at risk of famine and require immediate help "to save or sustain their lives".

"What I saw and heard during my visit to Somalia was distressing - women and children walk for weeks in search of food and water. They have lost their livestock, water sources have dried up and they have nothing left to survive on," Mr O'Brien said.

In northeast Nigeria, a seven-year uprising by the Islamic extremist group Boko Haram has killed over 20,000 people and driven 2.6 million from their homes.

A UN humanitarian coordinator said that malnutrition in the northeast is so pronounced that some adults are too weak to walk and some communities have lost all their toddlers.