UN humanitarian chief Stephen O'Brien told the UN Security Council the world was facing the "largest humanitarian crisis since the creation of the United Nations" with more than 20 million people facing starvation and famine across four countries.

He said without collective and coordinated global efforts, "people will simply starve to death".

Briefing the Security Council yesterday on his recent visit to Yemen, South Sudan, and Somalia, Mr O'Brien said the common factor in those countries was conflict. He stressed that the international community therefore had the "possibility to prevent and end further misery and suffering" in those countries at risk of famine.

Yemen has been engulfed in war since March 2015

Mr O'Brien said leadership on both sides of the conflict in Yemen promised to facilitate sustained humanitarian access, but "all parties to the conflict are arbitrarily denying sustained humanitarian access".

He said the humanitarian suffering in Yemen was caused by "the parties and proxies and if they don't change their behaviour now, they must be held accountable for the inevitable famine, unnecessary deaths and associated amplification in suffering that will follow".

Turning to South Sudan, Mr O'Brien described the situation in the country as "worse than it has ever been". He said the famine already declared in Unity State, in South Sudan, was "man-made" stressing that "parties to the conflict are parties to the famine, as are those not intervening to make the violence stop".

The humanitarian chief said in Somalia the current indicators mirrored the tragic picture of 2011 in the country when some 260,000 people died of famine.

He stressed that famine could be averted, but the donor agencies and nations needed to "invest in Somalia".

Mr O'Brien said the warning call and appeal for action by UN Secretary-General Antonio Guterres could not be understated.

Mr O'Brien said the UN and its partners were ready to scale up their response but needed "the access and the funds to do more" adding that it was possible to "avert these looming human catastrophes".