"The situation in Yemen - today, right now, to the population of the country - looks like the apocalypse," Mark Lowcock, the UN's undersecretary-general for humanitarian affairs and emergency relief coordinator, told Qatar-based broadcaster Al-Jazeera on Friday.
Lowcock said the "cholera outbreak is probably the worst the world has ever seen, with a million suspected cases until the end of 2017."
The UN official also expressed concern over "a terrible new epidemic" of diphtheria, which has killed scores of people and affected up to 500 others over the past few weeks.
He said the epidemic "is going to spread like wildfire across the country."
"Unless the situation changes, we're going to have the world's worst humanitarian disaster for 50 years."
Diphtheria is a contagious and potentially fatal bacterial infection, mainly characterized by a thick grey membrane at the back of the throat or nose, sore throat and fever. It can be prevented through vaccination.
The bacterial disease has been killing one in 10 Yemenis since the outbreak occurred in mid-August last year.
Cholera outbreak, which can cause potentially fatal dehydration if untreated, has also killed more than 2,000 people since April.
A nearly three-year-old war by Saudi Arabia on Yemen has virtually ruined the country’s health system, as critical shortages of medical supplies caused by Riyadh’s total blockade has aggravated the humanitarian situation for desperate Yemenis.
Saudi Arabia has been leading a deadly campaign against Yemen from the air, land, and sea since March 2015 in an attempt to reinstate former President Abd Rabbuh Mansur Hadi, a staunch ally of Riyadh, and to undermine the Houthi Ansarullah movement. Over the past two years, the Houthis have been running state affairs and defending Yemeni people against the Saudi aggression.
The offensive has, however, achieved neither of its goals despite the spending of billions of petrodollars and the joining of Saudi Arabia’s regional and Western allies in the war.
The war, which has so far killed at least 13,600 people, has also taken a heavy toll on the country’s infrastructure, destroying many schools and factories.