People in Somalia, Kenya, Djibouti and Ethiopia need food aid, water, new livestock and seeds, the Rome-based Food and Agriculture Organization said in a statement.
"Millions of people are on the brink of starvation in the Horn of Africa due to recent severe droughts coupled with the effects of past and ongoing conflicts," the agency said.
Across the continent, war-ravaged Congo is suffering the world's deadliest humanitarian crisis, with 38,000 people dying each month mostly from easily treatable diseases, according to a study published in Britain's leading medical journal.
Nearly 4 million people died between 1998-2004 alone, the indirect result of years of ruinous fighting that has brought on a stunning collapse of public health services, the study in the Lancet concluded.
FAO economist Shukri Ahmed said the Horn of Africa's dry season had begun and the rains forecast for March and April are not expected to be significant.
Normally, the herdsmen of the area would move from place to place for water and food for their livestock, but the recent drought had covered too large a swath of territory for them, Ahmed said.
"The whole area is affected," he said. "The situation is deteriorating."
The FAO is calling for domestic food purchases in areas where harvests are expected to be favorable and food aid imports elsewhere, U.N. spokesman Stephane Dujarric said at U.N. headquarters in New York.
The World Food Program is now feeding 1.2 million drought victims, "but fears this figure could more than double to 2.5 million," Dujarric said.
The food situation in Somalia and eastern Kenya is particularly serious, the FAO said. Ahmed said local newspapers, citing Kenyan medical officials, have reported at least 30 famine-related deaths.
The government of Kenya has said its efforts to distribute food to famine-stricken areas in its north have been hampered by the nation's nomadic culture and poor infrastructure. President Mwai Kibaki has declared a national disaster.
In Somalia, the secondary rainy season from October to December failed in most of the eight agricultural regions in the south, "resulting in widespread crop failure" that could be the worst in a decade, the agency said.
The country of 7 million that has not had an effective government since clan-based warlords overthrew dictator Mohamed Siad Barre in 1991. Warlords then turned on each other.
Nearly 150,000 people in Djibouti, or almost a fifth of the population, are believed to be facing food shortages because of drought, FAO said.
In Ethiopia, food shortages have been reported in the east and south, even though the prospects for the current harvest were favorable, the agency said. It said more than $40 million in aid was needed to stave off starvation.
About 3,000 U.N. soldiers guard the frontier between longtime enemies Ethiopia and Eritrea after a two-year war ended in 2000. Tensions have risen in recent weeks, with both countries massing troops along border and Eritrea restricting peacekeeping activities.