The worst humanitarian crisis in decades is happening right now

The United Nations is warning that the world is facing its worst humanitarian crisis since the end of World War II, with more than 20 million people facing starvation and famine in four countries.

The world body's humanitarian chief Stephen O'Brien called Friday for an urgent mobilization of funds — $4.4 billion by July — for northeastern Nigeria, Somalia, South Sudan and Yemen to "avert a catastrophe."

"Otherwise, many people will predictably die from hunger, livelihoods will be lost and political gains that have been hardwon over the last few years will be reversed," O'Brien said in his stark warning to the UN Security Council.

"Without collective and coordinated global efforts, people will simply starve to death. Many more will suffer and die from disease. Children stunted and out of school. Livelihoods, futures and hope will be lost."

He called war-wracked Yemen "the largest humanitarian crisis in the world," with two-thirds of the population, or 18.8 million people — three million more than in January — in need of assistance and more than seven million with no regular access to food.

The conflict in Yemen has left more than 7,400 people dead and 40,000 wounded since an Arab-state coalition intervened on the government's side against rebels in March 2015, according to UN figures.

In just the past two months alone, more than 48,000 people have fled fighting in the Arab world's poorest country, according to O'Brien, as it grapples with a proxy war fought by archrivals Iran and Saudi Arabia.

'Arbitrarily denying' access

During recent meetings, O'Brien said senior leaders in both parties agreed to provide continuous humanitarian access and respect international humanitarian law.

He noted that 4.9 million people received food assistance last month alone.

"Yet all parties to the conflict are arbitrarily denying sustained humanitarian access and politicize aid," he added.

"Already, the humanitarian suffering that we see in Yemen today is caused by the parties and proxies and if they don't change their behavior now, they must be held accountable for the inevitable famine, unnecessary deaths and associated amplification in suffering that will follow."

He noted that despite assurances from all parties that he would obtain safe passage to the flashpoint city of Taiz, he was in fact denied access and came under gunfire after retreating to a short distance away.

A total of $2.1 billion are needed to reach 12 million people with life-saving assistance and protection in Yemen this year, according to O'Brien, who noted that just six percent of those funds have been received so far.

He announced that a ministerial-level pledging event for Yemen will take place in Geneva on April 25, to be chaired by UN chief Antonio Guterres.

Politics behind 'man-made famine'

During his visit last week to South Sudan, the world's youngest nation, O'Brien said he found a situation that is "worse than it has ever been."

"The famine in South Sudan is man-made," he added.

"Parties to the conflict are parties to the famine — as are those not intervening to make the violence stop."

He said more than 7.5 million people need assistance, an increase of 1.4 million fro last year. And some 3.4 million people are displaced, including nearly 200,000 who have fled South Sudan since January alone.

More than half the population of Somalia — 6.2 million people — need humanitarian assistance and protection, including 2.9 million at risk of famine.

Nearly one million children under the age of five will be "acutely malnourished" this year, according to the humanitarian chief, who also visited the country.

Read more: Uganda has been a welcoming place for South Sudan's refugees

"What I saw and heard during my visit to Somalia was distressing — women and children walk for weeks in search of food and water," O'Brien said.

"They have lost their livestock, water sources have dried up and they have nothing left to survive on. With everything lost, women, boys, girls and men now move to urban centers."

In northeastern Nigeria, O'Brien said 10.7 million people need humanitarian aid, including 7.1 million people who are "severely food insecure." 

The humanitarian emergency afflicting the area was triggered by the Boko Haram insurgency, which erupted in Nigeria in 2009. Poor governance and climate change have also been powerful contributors to the crisis.

The conflict, which has left around 20,000 people dead and forced more than 2.6 million others to flee their homes, has aggravated an already difficult humanitarian situation in one of the poorest regions of the world.