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.