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.
In Congo, the majority of deaths were due to disease rather than violence, but war has cut off or reduced access to health services for millions in the impoverished nation the size of Europe, according to the study published Friday.
Most deaths reported were due to "preventable and easily treatable diseases," the study said. Malaria, diarrhea, respiratory infections and malnutrition topped the list.
Major fighting ended in Congo in 2002 but the situation remains dire because of continued insecurity, poor access to health care and inadequate international aid. The problems are particularly acute in eastern Congo.
"Rich donor nations are miserably failing the people of (Congo), even though every few months the mortality equivalent of two southeast Asian tsunamis plows through its territory," the study said.
Backed by about 15,000 U.N. peacekeepers, Congo's government is struggling to re-establish authority across the country ahead of elections expected later this year, the first in decades. Militiamen still roam huge swaths of the east, formerly controlled by several different rebel groups whose leaders have been allotted top government posts.
The study was based on a survey of 19,500 households across the country of 60 million between April and July 2004. Health Ministry workers and staff of the aid group International Rescue Committee conducted the interviews.
The results showed Congo's monthly mortality rate was 40 percent higher than the average for sub-Saharan Africa: 2.1 deaths per 1,000 people, or the equivalent of 1,200 fatalities per day, compared with a continental average of 1.5 deaths per 1,000.
Mortality rates were highest in Congo's eastern provinces, which have been wracked by fighting and lawlessness for a decade. There, death rates were 93 percent higher than the sub-Saharan Africa average.
"The persistently high mortality in ... Congo is deeply disturbing and indicates that both national and international efforts to address the crisis remain grossly inadequate," the report said.
The survey is the fourth of its kind conducted in Congo, Africa's third-largest nation. The International Rescue Committee conducted three earlier surveys, the last of which in 2004 said that six years of conflict had claimed 3.8 million lives, mostly due to disease and food shortages.
Congo's government dismissed the report.
"I consider that a big lie," Information Minister Henri Mova Sakanyi said. "These figures are very exaggerated. All over the world, people die of disease, it's not just Congo," Sakanyi told The Associated Press.
"It's known that (aid) agencies have often played with the figures ... to get financial support," the minister added.
The Lancet study said the deaths counted were "excess" deaths that would not have occurred if the situation in Congo was normal.