The Secretary-General for Humanitarian Affairs and Emergency Relief told the United Nations Security Council the people of Yemen are suffering deprivation, disease and death while the world watches on.
Stephen O'Brien believes it's not the unforeseen or coincidental result of forces beyond other nations' control; instead he says it's the direct consequence of actions by parties to, and supporters of, the conflict, and inaction by the international community.
Mr O'Brien warns the situation in Yemen continues to spiral downwards towards what he calls "total social, economic and institutional collapse".
"Yemen now has the ignominy of being the world's largest food security crisis with more than 17 million people who are food insecure, 6.8 million of whom are one step away from famine. Crisis is not coming, it is not even looming: it is here today, on our watch, and ordinary people are paying the price."
Stephen O'Brien says some of Yemen's population are being forced to take desperate action.
"Over one million civil servants have not been paid for months, affecting more than eight million people, pushing more and more families towards poverty and starvation. Families are increasingly marrying off their young daughters to have someone else care for them, and often use the dowry to pay for basic necessities."
Yemen also faces a cholera outbreak which official sources say has killed more than 400 people in the past month.
Tens of thousands of suspected cases have also been identified.
Authorities declared a state of emergency in the capital, Sana'a, the area hardest-hit by the spread of the potentially fatal water-borne disease.
The UN regards the situation in Yemen as an "unprecedented humanitarian crisis", but says only a quarter of the almost three-billion-dollars it needs to fund aid for the country has been raised.
Two years of conflict between Houthi rebels and government forces backed by a Saudi-led military alliance has killed 10,000 people and displaced millions.
Armed groups al-Qaeda and the self-proclaimed Islamic State are exploiting the war to increase their presence, prompting airstrikes by the United States.
Peace eludes what before the war was already the poorest nation on the Arabian Peninsula, and the UN wants to prevent further escalation.
It's urging against a new threat by the Arab alliance to help government troops retake territory held by Houthi rebels, including the Red Sea port of Hodeidah, a crucial access point for food and humanitarian imports.
Yemen's envoy to the UN, Ismail Ould Cheikh Ahmed, also says the conflict must not be allowed to expand.
"I urge the Council to strongly convey to the parties that they need to engage immediately with the United Nations to agree on steps to avoid further bloodshed, to halt the slide towards famine and to recommit to a peaceful end to the war. The humanitarian crisis and the threat of famine are entirely man-made. If the conflict stops Yemenis and their partners, and the international community, have the capacity and the will to rebuild the country."
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