The UN humanitarian chief pleaded for help Friday as he warned of the "largest humanitarian crisis since the creation of" the global body in 1945.Stephen O'Brien told the U.N. Security Council that "without collective and coordinated global efforts, people will simply starve to death" and "many more will suffer and die from disease.""It is all preventable," he said. "It is possible to avert this crisis, to avert these famines, to avert these looming human catastrophes."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."U.N. and food organizations define famine as when more than 30 percent 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."He called war-wracked Yemen "the largest humanitarian crisis in the world," with two thirds of the population, or 18.8 million people -- three million more than in January -- in need of assistance and more than seven million with no regular access to food.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 politicize aid," he said, warning if that behavior 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 percent 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 U.N. 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."O'Brien said more than 7.5 million people need aid, up by 1.4 million from last year, and about 3.4 million South Sudanese are displaced by fighting including almost 200,000 who have fled the country since January. "More than one million children are estimated to be acutely malnourished across the country, including 270,000 children who face the imminent risk of death should they not be reached in time with assistance," he said. "Meanwhile, the cholera outbreak that began in June 2016 has spread to more locations."In northeast Nigeria, a seven-year uprising by the Islamic extremist group Boko Haram has killed more than 20,000 people and driven 2.6 million from their homes. A U.N. humanitarian coordinator said last month that malnutrition in the northeast is so pronounced that some adults are too weak to walk and some communities have lost all their toddlers.O'Brien also called attention to drought-affected Kenya, where he said 2.7 million people already faced food insecurity, a number he said could rise to 4 million by April.