From the BBC:
More than 10 million people are thought to be affected across the region.
The UN now classifies large areas of Somalia, Ethiopia, Djibouti and Kenya as a crisis or an emergency.
Charity Save the Children says drought and war in Somalia has led to unprecedented numbers fleeing across the border into Kenya, with about 1,300 people arriving every day.
Three camps at Dadaab, just inside Kenya, are home to well over 350,000 people, but they were built to hold just 90,000 and are severely overcrowded.
A prolonged failure of rains, which began in late 2010, is now taking its toll.
The UN's Office for the Co-Ordination of Humanitarian Affairs (Ocha) warns that the situation is continuing to deteriorate, and the number of people in need will continue to increase.
The numbers now affected are huge, Ohca says: 3.2m in Ethiopia, 3.2m in Kenya, 2.6m in Somalia and more than 100,000 in Djibouti.
Every month during 2011, about 15,000 Somalis have fled their country, arriving in Kenya and Ethiopia, according to Ocha.
While conflict has been a fact of life for them for years, it is the drought that has brought them to breaking point. Many have walked for days, are exhausted, in poor health, desperate for food and water.
Nearly one third of all children in the Juba region of Somalia are acutely malnourished, while in parts of Ethiopia the figure is even higher, the UN research says. Parts of Uganda are also suffering from the drought.