SPONSORED | The Gauteng department of human settlements, together with the Gauteng Partnership Fund,.
Some 20million people live in hard-hit areas where harvests have failed and malnutrition rates are increasing, particularly among young children, the UNHCR said.
In South Sudan alone, where the UN declared famine in some areas in February, "a further 1million people are now on the brink of famine", UNHCR spokesman Adrian Edwards said.
"We are raising our alarm level further by 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 there are also greater numbers of South Sudanese refugees fleeing to Sudan and Uganda, according to the UNHCR,
A preventable humanitarian catastrophe - possibly worse than that of 2011 when 260000 people died of famine in the Horn of Africa, half of them children - "is fast becoming an inevitability", Edwards said.
"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," he said. "A repeat must be avoided at all costs."
UNHCR is scaling up its operations but is stymied by a severe funding shortfall, with some of the country programmes only funded at between 3% and 11%, Edwards said.
Overall the UN has appealed for $4.4-billion (about R61-billion) for the four countries, but has received less than $984-million or 21% to date, Jens Laerke of the UN Office for Coordination of Humanitarian Affairs said.