Yemen is facing a mass famine that will affect millions of lives unless the Saudi-led coalition ends its blockade and allows aid deliveries into the country, the UN aid chief warned Wednesday.

Mark Lowcock, the UN under-secretary-general for humanitarian affairs, said that unless the borders are re-opened to aid shipments, "it will be the largest famine the world has seen for many decades, with millions of victims."

The UN official spoke to reporters after briefing the Security Council during a closed-door session on the crisis in Yemen, where the coalition has been waging a military campaign against Huthi rebels since March 2015.

Lowcock told the council that UN humanitarian flights must be allowed to resume to the rebel-held capital Sana and to the government-controlled city of Aden.

He said there must be "immediate access to all sea ports" for deliveries of fuel, food and other vital supplies -- as well as assurances from the coalition that there will be no further disruption.

The coalition shut down Yemen's borders and halted aid deliveries on Monday in response to a missile attack by Yemen's Huthi rebels that was intercepted near the Riyadh airport.

The United Nations, which had already listed Yemen as the world's number one humanitarian crisis, has raised alarm and recalled that the situation was already "catastrophic" in the country.

Some 17 million Yemenis are in desperate need of food, seven million of whom are at risk of famine and cholera causing more than 2,000 deaths.

On Tuesday, a Red Cross shipment of chlorine tablets, which are used for the prevention of cholera, was blocked at Yemen's northern border, the International Committee for the Red Cross said.

The Saudi-led Arab military coalition intervened in Yemen in March 2015 to support President Abedrabbo Mansour Hadi after the Huthis forced him into exile.