The United Nations refugee agency (UNHCR) has once again sounded alarm bells as it warned of the risk of "mass deaths" as an arc of hunger and violence looms across the Horn of Africa and three African countries.

Drought and famine is threatening 20million people across Somalia, South Sudan, Yemen, and north-east Nigeria, and almost 1.4million children are at imminent risk of death this year, according to the UN. Those at risk include 4.2million refugees.

UNHCR spokesman Adrian Edwards warned that what was "an avoidable humanitarian crisis" was "fast becoming an inevitability" due to drought, crippling conflict and "severe" funding shortfall.

Speaking to press in Geneva, Edwards warned the dangerous combination of factors risked making the current crisis worse than a similar drought in 2011.

Exacerbated by years of civil war, the world's last famine killed 260,000 people in the Horn of Africa.

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

UN humanitarian agency spokesman Jens Laerke outlined how only $984 million, or 21%, of the $4.4bn (£3.5bn) the UN appealed for to address the famine crisis has so far been received.

The arc of hunger extends from the red soil of Nigeria in the west, where Boko Haram's six-year jihadist insurgency has forced two million people to flee their homes, to Yemen's white deserts in the east, where warring factions block aid while children starve as the nation already experiences the world's largest humanitarian crisis.

Between them lie Somalia's parched sands and the swamps of oil-rich South Sudan, where starving families fleeing three years of civil war survive on water-lily roots. Parts of South Sudan are already suffering famine, the first in six years.

In Somalia, the UN says more than half the 12 million population need aid.