The United Nations says it needs to raise $5.6 billion—most of it by the end of March—to avert a famine affecting 20 million people in Yemen and several African countries. U.N. Secretary-General António Guterres said Wednesday that war and climate change have left millions at risk of starvation in a “world of plenty.”

Secretary-General António Guterres: “I’m here with my colleagues to draw the world’s attention to the fact that today more than 20 million people in South Sudan, Somalia, Yemen and northeast Nigeria are going hungry and facing devastating levels of food insecurity. Famine is already a reality in parts of South Sudan. Unless we act now, it is only a matter of time until it affects other areas and other countries. We are facing a tragedy. We must avoid it becoming a catastrophe. This is preventable, if the international community takes decisive action.”

Hardest hit is Yemen, where a U.S.-backed, Saudi-led bombing campaign and naval blockade has left more than 14 million people hungry. Among them are nearly a half-million children who face severe acute malnutrition.