"This warning is in light of droughts that are also affecting many neighboring countries and a funding shortfall that has become so severe that an avoidable humanitarian crisis in the region, possibly worse than that of 2011, is fast becoming an inevitability," the UN Refugee Agency, UNHCR, said on Tuesday.

Speaking in Geneva, UN Office for the Coordination of Humanitarian Affairs (OCHA) spokesman Jens Laerke said: "More than 20 million people in north-eastern Nigeria, South Sudan, Somalia and Yemen are experiencing famine or a credible risk of famine."

Laerke said in order to avert a humanitarian catastrophe in the four countries, UN humanitarian operations urgently required $4.4 billion.

"The humanitarian requirements globally are at unprecedented levels with a current global request of $22.8 billion to meet the humanitarian needs of nearly 100 million people across the world.

“Some $2.7 billion has been committed so far," Laerke said.