Millions of people will die in Yemen, in what will be the world's worst famine crisis in decades, unless a Saudi-led military coalition ends a devastating blockade and allows aid into the country, the United Nations has warned.

The Saudi-led alliance fighting Houthi rebels in Yemen tightened its air, land and sea blockade of the country after a ballistic missile was fired on Saturday towards the Saudi capital, Riyadh.

Since then, the country's already inflated food and fuel prices have skyrocketed, while flights delivering much-needed humanitarian aid have been prevented from landing.

After briefing the UN Security Council on Wednesday, Mark Lowcock, the UN's humanitarian chief, said the move will worsen a "catastrophic" humanitarian crisis that has pushed millions to the brink of famine and has caused a mass cholera epidemic.

"Ihave told the Council that unless those measures are lifted ... there will be a famine in Yemen," Lowcock, who visited Yemen late in October, told reporters on Wednesday.

"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," he added.

"It will be the largest famine the world has seen in many decades, with millions of victims."

'Guarded response'

Despite Lowcock's stark warning, the Security Council, which strongly condemned the firing of the missile by the Houthi rebels, announced no immediate action over the blockade.

Italian Ambassador Sebastiano Cardi, who holds the council's presidency, expressed concern about the "dire humanitarian