(AFP) - The United Nations warned Tuesday of a growing risk of mass deaths from starvation among people living in conflict and drought-hit areas of the Horn of Africa, Yemen and Nigeria.

An "avoidable humanitarian crisis... is fast becoming an inevitability", as the UN faces a "severe" funding shortfall to help people affected by famine, said UN refugee agency spokesman Adrian Edwards.

UNHCR's operations in famine-hit South Sudan, and in Nigeria, Somalia and Yemen, which are on the brink of famine, are funded at between just three and 11 percent, he told reporters in Geneva.

As a whole, the United Nations has requested $4.4 billion to address the crisis in the four countries, but has so far received only $984 million, said UN humanitarian agency spokesman Jens Laerke said.

The current crisis risks becoming worse than the 2011 drought in the Horn of Africa that killed more than 260,000 people in Somalia alone, Edwards said.

"A repeat must be avoided at all costs," he said.

More than 20 million people across Nigeria, South Sudan, Somalia and Yemen, are in areas hit by drought and are experiencing famine or are at high risk of famine, according to UN numbers.

- 'Descent into disaster'

"It is of immediate urgency that more funds are committed to avert a further descent into disaster in these acute crises," Laerke said.

In conflict-ravaged South Sudan, where the UN warned in February that fighting, insecurity, lack of access to aid and the collapsing economy had left 100,000 people facing starvation, "a further one million people are now on the brink of famine," Edwards said.

And in Yemen, which is already experiencing the world's largest humanitarian crisis, 17 million people, or around 60 percent of the war-torn country's population, are going hungry.

In northern Nigeria meanwhile, seven million people are currently struggling with food insecurity, with the situation particularly bad in the northeast of the country, a stronghold of Boko Haram jihadists.

The situation is also "very, very dire" in troubled Somalia, said David Hermann, who coordinates operations in the country for the International Committee of the Red Cross.

"The response should happen now, because if it doesn't happen now... people are going to die from starvation," he told reporters.

Edwards said the growing food insecurity was pushing more and more people to leave their homes across the region, with food needs cited as the main factor causing displacement in most locations in Yemen and South Sudan for instance.

"In Sudan, for example, where our initial estimate was for 60,000 arrivals from South Sudan this year, we are in the process of revising the expected total upwards to 180,000," he told reporters.

He said the lack of funding meant less food distributed to those who need it most: the more than four million refugees in the region, most of whom are children.

"With no money to buy food, rations... are being cut," he said, adding that in Djibouti rations have been cut by 12 percent, in Ethiopia, Tanzania and Rwanda by between 20 and 50 percent, and in Uganda by up to 75 percent.

This can have dramatic consequences, he warned, since "many refugees are without full access to livelihoods and agriculture or food production and their ability to take matters into their own hands and help themselves is limited."

- Lake Chad 'crisis' -

Meanwhile the UN Food and Agriculture Organization warned that at least seven million people were at risk of severe hunger in the Lake Chad region, calling for "urgent support".

The region straddles northeast Nigeria, the far north of Cameroon, western Chad and southeast Niger. The countries share a border on the shallow, freshwater lake.

"The crisis .. is rooted in decades of neglect, lack of rural development and the impact of climate change," FAO Director-General Jose Graziano da Silva said in a statement.

© Agence France-Presse