Saudi Arabia tightened its blockade on Yemen after Houthi rebels, who have taken over the national government and its assets, fired a ballistic missile last week toward an airport in Riyadh. The Saudis were able to intercept the projectile, but the event has rattled the region politically.
Saudi Arabia has carried out an air campaign against the Houthi rebels since 2015, after civil war broke out and the internationally recognized government was forced out of the capital, Sanaa.
Since Monday, Riyadh has prevented aid agencies from landing planes in the country and docking at Yemen's ports, worsening an already dire humanitarian situation, said UN humanitarian chief Mark Lowcock on Wednesday.
He explained he had put proposals to the UN Security Council to ensure the resumption of aid deliveries, and if they were not achieved, Yemen would descend into famine.
"It will not be like the famine that we saw in South Sudan earlier in the year, where tens of thousands of people were affected. It will not be like the famine which cost 250,000 people their lives in Somalia in 2011. It will be the largest famine the world has seen for many decades, with millions of victims."
Riyadh announced it was temporarily closing all access into the country by ground, air and sea in response to the missile, but said it would take into consideration "the continuation of the entry and exit of humanitarian supplies and crews," according to the state-run SPA news agency.
But Lowcock said Wednesday that not a single UN plane had been able to land in the country, and that he believed no other humanitarian agency had been able to access the country.
He did not say how fast famine might take hold in Yemen, but he explained the lack of food would lead to a whole range of medical complications.
"What kills people in famine is infections, or measles, or respiratory tract problems, or a cold. Because their bodies have consumed themselves, reduced totally the ability to fight off things which a healthy person can fight off."
Dire cholera epidemic
Lowcock's warning comes as Yemen is facing one of the world's worst cholera outbreaks in modern history.
The World Health Organization says there have been more than 900,000 suspected cases of cholera in Yemen since late April, many of them children, and that there are expected to be 1 million cases before the end of the year.
More than 2,000 people have died so far in the cholera outbreak alone.
Parts of Yemen have been on the brink of famine and images of emaciated children have emerged during Yemen's war, which has raged now for more than two years. And more than half of the country's medical facilities have closed, cutting off much of the population from essential healthcare.
The WHO reports that more than 8,600 people have died in conflict-related deaths and 49,963 people have been injured since the since the start of the war in March 2015 until mid-September.
Those numbers are based on reports from health facilities, and the actual number of casualties is believed to be higher.