The world faces the largest humanitarian crisis since the United Nations was founded in 1945 with more than 20 million people in four countries facing starvation and famine, the UN humanitarian chief has said.

Stephen O'Brien on Saturday told the UN Security Council that "without collective and coordinated global efforts, people will simply starve to death" and "many more will suffer and die from disease."

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He urged an immediate injection of funds for Yemen, South Sudan, Somalia and northeast Nigeria plus safe and unimpeded access for humanitarian aid "to avert a catastrophe."

"To be precise," O'Brien said, "we need $4.4 billion by July."

Without a major infusion of money, he said, children will be stunted by severe malnutrition and won't be able to go to school, 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 per cent of children under age 5 suffer from acute malnutrition and mortality rates are two or more deaths per 10,000 people every day, among other criteria.

"Already at the beginning of the year we are facing the largest humanitarian crisis since the creation of the United Nations," O'Brien said. "Now, more than 20 million people across four countries face starvation and famine."

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 and don't know where their next meal will come from. "That is three million people more than in January," he said.

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

During his recent visit to Yemen, O'Brien said he met senior leaders of the government and the Shiite Houthi rebels who control the capital Sanaa, and all promised access for aid.

"Yet all parties to the conflict are arbitrarily denying sustained humanitarian access and politicise aid," he said, warning if that behaviour doesn't change now "they must be held accountable for the inevitable famine, unnecessary deaths and associated amplification in suffering that will follow."

For 2017, O'Brien said 2.1 billion is needed to reach 12 million Yemenis "with life-saving assistance and protection" but only 6 per cent has been received so far. He announced that Secretary-General Antonio Guterres will chair a pledging conference for Yemen on April 25 in Geneva.

The UN humanitarian chief also visited South Sudan, the world's newest nation which has been ravaged by a three-year civil war, and said "the situation is worse than it has ever been."

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