The risk of mass starvation in four countries - Nigeria, Somalia, South Sudan and Yemen - is rising rapidly due to drought and conflict, according to the UN refugee agency.
About 20 million people live in hard-hit areas where harvests have failed and acute malnutrition rates are increasing, particularly among children, it said.
In South Sudan, where the United Nations declared famine in some areas in February, "a further 1 million people are now on the brink of famine", UNHCR spokesman Adrian Edwards said.
"We are raising our alarm level further by today warning that the risk of mass deaths from starvation among populations in the Horn of Africa, Yemen and Nigeria is growing," Edwards told a news briefing.
"This really is an absolutely critical situation that is rapidly unfolding across a large swathe of Africa from west to east."
People are on the run within their countries and greater numbers of South Sudanese refugees are fleeing to Sudan and Uganda, the U.N. High Commissioner for Refugees (UNHCR) said.
A preventable catastrophe, possibly worse than that of 2011 when 260,000 people died of famine in the Horn of Africa, half of them children, "is fast becoming an inevitability."
"Always the problem that we have with humanitarian crises in sub-Saharan Africa is that they tend to get overlooked until things are too late," Edwards added.
"A repeat must be avoided at all costs. There's acute malnutrition, very high rates, if you don't help people with worsening rates of malnutrition, people die."
Seven million people in north-east Nigeria and the Lake Chad basin are suffering from food insecurities.
UNHCR is scaling up its operations but has a funding shortfall, with some country programmes only funded at between 3 and 11 per cent.
Overall the United Nations has appealed for $4.4 billion for the four countries but has received less than $984 million to date.
The International Committee of the Red Cross (ICRC) warned in late March that the world had three to four months to stop starvation in the four countries.
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