GENEVA (Reuters) - The risk of mass starvation in four countries - northeast Nigeria, Somalia, South Sudan and Yemen - is rising rapidly due to drought and conflict, the U.N. refugee agency said on Tuesday.

Zeinab, 14, sits as she holds her nephew at a camp for internally displaced people from drought-hit areas in Dollow, Somalia. REUTERS/Zohra Bensemra

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,” he said.

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”, Edwards said.

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“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.”

“There’s acute malnutrition, very high rates, if you don’t help people with worsening rates of malnutrition, people die.”

Seven million people in northeast Nigeria and the Lake Chad basin are suffering from food insecurity, Edwards said.

Food security in 4 countries is expected to continue to deteriorate until at least mid-year, he said.

UNHCR is scaling up its operations but has a funding shortfall, with some country programmes only funded at between 3 and 11 percent, he said.

Overall the United Nations has appealed for $4.4 billion for the four countries but has received less than $984 million to date, Jens Laerke of the U.N. Office for the Coordination of Humanitarian Affairs said.

In northeast Nigeria, especially Borno state, aid workers had “almost zero access” a year ago, due to Boko Haram militants, Laerke said.

“Now that access is opening up. That is one of the reasons why the numbers have grown. Because as we have pushed into these areas we have simply discovered more and more need of an extreme nature including extreme food insecurity so that there is a risk of famine,” he said.

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.

David Hermann, ICRC operations coordinator for Somalia, told the briefing: “If it doesn’t happen now, there will be no escaping a situation that this country already experienced a few years ago, meaning probably people are going to die from starvation and disease.”