The number of children in Somalia suffering from acute malnutrition this year has "shot up" by 50%, according to estimates by the United Nations' children's agency, Unicef.

Somalia is facing the "largest humanitarian crisis in history of the United Nations", as millions are at risk of starvation and famine across the Horn of Africa, Nigeria and Yemen, due to lack of rain and prolonged insecurity. In Somalia, the UN also says more than half the 12million population need aid.

Unicef, on 2 May, revised upwards its estimate of the number of acutely malnourished children in Somalia to 1.4million.

"Severely malnourished children are nine times more likely to die of killer diseases like cholera and acute watery diarrhoea and measles, which are spreading," the agency said.

Drought and famine is threatening 20million people across Somalia, South Sudan, Yemen, and north-east Nigeria. Those at risk include 4.2million refugees.

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

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.