The United Nations has launched a $425m appeal to help feed 8m people in urgent need of assistance across East Africa.
Nomads are left destitute when their animals die
"Nowhere else on earth is so much at stake as in Africa," said the UN's most senior aid official, Jan Egeland. "It is here where most lives are at stake."
Across five countries - Djibouti, Ethiopia, Eritrea, Kenya and Somalia - 15m people are estimated to have been affected by the drought.
Rains have begun to fall but this will not end the crisis, aid workers say.
Mr Egeland said he did not know how many people had died from hunger but said it could be in the thousands, even if they were not starving to death.
"These children dying of diarrhoea, these women dying from preventable diseases, these elderly dying too early, die because they have been weakened by malnutrition, they have been weakened by lack of water, lack of sanitation and preventable disease kills them at intolerable levels," he said.
Many thousands of animals have already succumbed to hunger, leaving nomads, who rely on animals as a store of wealth as well as source of food, destitute.
Mr Egeland also warned that the food crisis could worsen unrest in Somalia, which has not had an effective central government for 15 years and which is awash with weapons.
"Angry, hungry men with Kalashnikovs in search of food somewhere will lead to more conflict," he warned.
Most of the money needed - $327m - was for Somalia, he said.
After two failed seasons, the rain is again falling across the region.
"But this does not mean the end of the crisis - Far from it: initially the rain will exacerbate an already fragile situation," warned aid agency Oxfam.
"In fact, in the short term they could make the situation worse by spreading disease and blocking access for food aid."
Rain has already caused flooding and damaged roads in some parts of north-eastern Kenya.
Some 3,000 people had to leave their homes in the north-east of the country after a river burst its banks.
Aid workers were having trouble getting to the area because of damage to roads, an official told Reuters news agency.
Oxfam warned last month that north-eastern Kenya could take 15 years to recover from the effects of drought.