Story highlights UN humanitarian chief says Riyadh has prevented aid agencies from landing planes in the country

Warning comes as Yemen is facing one of the world's worst cholera outbreaks in modern history

(CNN) The UN's humanitarian chief has sent a chilling warning that Yemen is facing the world's worst famine in decades in which millions could die, if Saudi Arabia continues to block aid flowing into the war-torn nation.

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.

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