The United Nations has warned that the world is facing the largest humanitarian crisis since 1945 with starvation and famine in Yemen, South Sudan, Somalia and northeastern Nigeria.

Stephen O'Brien, the British UN humanitarian chief, stated on Friday that more than 20 million people across those four countries could die.

"Without collective and coordinated global efforts, people will simply starve to death," he said at the UN Security Council, adding that "many more will suffer and die from disease".

1.4 million children are at imminent risk of death.

The worst-hit country is Yemen, where two-thirds of the population - 18.8 million people - need aid and more than seven million do not know where their next meal will come from.

Mr O'Brien said that is now three million more chronically hungry people in Yemen than in January.

A peace deal between South Sudan and Sudan signed in August 2015 has failed, and clashes last July between the two forces set off further violence, killing tens of thousands of people and forcing 3.1 million to flee their homes.

An estimated 100,000 people in the country are experiencing famine, and one million others are on the brink of starvation.