The drought has already killing thousands.
Jan Egeland, the UN humanitarian chief, said at the appeal's launch in Kenya, one of the worst hit nations: "Nowhere else on earth is so much at stake as in Africa. It is here where most lives are at stake."
Since late 2005, east Africans have been facing hunger and hardship, and losing livestock, due to a drought one aid agency, Oxfam, said on Friday will take 15 years to recover from.
Rain has finally fallen in some parts in recent days.
But aid officials say it is too late to alleviate the suffering. Egeland said the number in urgent need could rise to 15 million, adding that if a similar number in other regions faced hunger the world would be scandalised.
"It would be evident if, say, all of Scandinavia faced collective starvation, the world would really respond.
"It would be evident if, say, all of Scandinavia faced collective starvation, the world would really respond"
UN humanitarian chief
"If all of northern Iraq was facing massive starvation, I think the world would really respond. If Kosovo and Bosnia again faced starvation, I think the world would massively respond."
Egeland said the drought had decimated livestock and its effects were killing many, particularly children.
Disease and malnutrition
He said: "People are not dying yet in large numbers because of hunger itself, they're dying however because of associated disease and malnutrition.
"Malnourished children die now in very high numbers, it's certainly in the thousands, probably in tens of thousands across the Horn of Africa," he added, responding to a question about how many lives had been lost due to the drought.
Of the $426 million sought, Egeland said the bulk - $327 million - was for Somalia, a Horn of Africa nation overrun by feuding regional commanders for the past 15 years.
"If we fail to invest in Somalia today, there will be more conflict. Because if it's one thing that Somalia's full of, it's small arms," Egeland said.
"Angry, hungry men with Kalashnikovs in search of food somewhere will lead to more conflict and a collapse of the whole project of establishing a viable state."
Egeland pleaded for investment in livestock recovery and the re-establishment of communities to break a cycle of drought and hunger he said was now afflicting Africa every two years, compared to every decade before.
Oxfam, based in the UK, said the rains in some parts of east Africa were a mixed blessing, helping crops and grazing, but also blocking aid routes and harming weak animals further.
Andrew Featherstone, the Oxfam regional manager, said: "Initially the rain will exacerbate an already fragile situation."
The hunger in Somalia could
further destabilise the country
Oxfam said the few surviving animals were frail and unable even to shake rainwater from their coats: "Large proportions of the exhausted and malnourished livestock could die due to the rains and change of temperature."
The drought has hit Kenya, Somalia and Ethiopia hardest, but also Djibouti, Eritrea, Burundi, Uganda and Tanzania.
Egeland, too, sounded a warning on the rains, saying deceptive good rains in the region's unofficial capital, Nairobi, should not lead to complacency.
"It is far too little in the vulnerable areas to be any promise of anything yet. In some areas, it's been far too much too suddenly, a flood, then the water disappears."