The United Nations has declared a famine in two parts of southern Somalia, as the Horn of Africa suffers its worst drought in decades.

The world body on Wednesday said a famine exists in the Bakool and Lower Shabelle regions, and U.N. chief Ban Ki-moon warned it could spread if immediate action is not taken to address extreme food shortages.

Mr. Ban told reporters in New York that Somalis are dying at an “appalling rate” and that every delay will cost more lives.

The U.N. humanitarian chief in Somalia, Mark Bowden, said 3.7 million Somalis, nearly half the population, is in crisis, and tens of thousands of Somalis have already died because of malnutrition.

The U.N. said $300 million in aid is needed during the next two months in Somalia alone.

The U.N. World Food Program on Wednesday elevated the food crisis in East Africa to its highest global priority. It said with more than 11 million people in need of food aid, a rapid scale-up in assistance is needed to avert the widespread loss of life.

U.S. Secretary of State Hillary Clinton announced Wednesday that Washington will add $28 million in aid to the more than $431 million in assistance it has already provided for the region.

The U.N. declares a famine when malnutrition rates in an area are higher than 30 percent, when more than two people per 10,000 are dying each day, and when many cannot get access to food and other essentials.

The extended drought in Somalia is prompting thousands of Somalis to cross borders in Kenya and Ethiopia each day in search of food.