More than 300,000 children in the Horn of Africa are severely malnourished "and in imminent risk of dying" because of drought and famine, the head of the U.N. children's agency said Friday. The United Nations says tens of thousands of people already have died in Somalia, Kenya, Ethiopia and Djibouti and has warned that the famine hasn't peaked. More than 12 million people in the region need food aid, according to the U.N.. "The crisis in the Horn of Africa is a human disaster becoming a human catastrophe," UNICEF Executive Director Anthony Lake told reporters. In Somalia alone, he said, 1.4 million children are affected, an estimated 390,000 are suffering from malnutrition, and nearly 140,000 in the south-central region are facing imminent death from "severe acute malnutrition." Somalia, which has been engulfed in conflict for nearly two decades, has been hardest-hit with famine in five regions. The southern and central parts of the country, which are mainly under control of al-Shabab extremists, have been worst affected because of the Islamic group's refusal to allow key humanitarian organizations to deliver aid. Among the groups that have been blocked is the U.N. World Food Program, the world's major aid provider. While UNICEF, the Red Crescent and other organizations are working in the south-central region, providing food and water and operating nutrition centers, Lake predicted "the crisis will get worse." "Let me warn that by the next rainfalls in October, we project that all of central and south Somalia will suffer the same extreme food and nutrition crises as is the case in the worst areas there today, with twice as many children - almost 300,000 - in imminent peril," he said. Donors have contributed over $1 billion to help famine victims, but U.N. Secretary-General Ban Ki-moon appealed again Friday - World Humanitarian Day - for $1 billion more. "Humanitarians are on the ground saving lives," he said. "But we are still not reaching all the people who need our help, and the crisis has still not peaked." Lake said children are the most vulnerable and suffer most in disasters like the current drought and famine. "In many ways this is a children's crisis and their plight demands our most urgent, bold and sustained response," he said. He added: "We are in a fight against time."