The Saudi-led military coalition must fully lift its blockade on Yemen, where 7 million to 8 million people are "right on the brink of famine," United Nations humanitarian affairs chief Mark Lowcock said Friday.

"That blockade has been partially but not fully wound down. It needs to be fully wound down if we are to avoid an atrocious humanitarian tragedy involving the loss of millions of lives, the like of which the world has not seen for many decades," he said, as reported by Reuters.

Earlier this month, Saudi Arabia's powerful Crown Prince Mohammed bin Salman said arming Yemen's Houthis could be considered an act of war, provoking a heated war of words with Tehran.Iran denies it is supplying the Houthis with arms. Saudi Arabia and a coalition of mainly Sunni Arab allies launched airstrikes in March 2015 on the Houthis and later sent ground troops to support pro-government forces.

The U.N. Monday urged the Saudi-led coalition to do "much more" to ease the blockade impeding shipments of aid and fully reopen the key rebel-held Red Sea port of Hodeida.

The coalition has allowed some supplies to reach rebel-held Sanaa and the Saleef Red Sea port, also in the Houthis hands. But little aid has entered through Hodeida, the main conduit for U.N.-supervised deliveries of food and medicine.

U.N. officials say Yemen could face the world's largest famine in decades unless the blockade is lifted. Millions in the country are at immediate risk if food aid and the supply of fuel for pumping clean water are interrupted, he said.

About 7 million people in Yemen, out of a population of 27 million, depend entirely on food aid, and 4 million rely on aid groups for clean water. More than 2,000 people have died of cholera in Yemen this year, adding to the 8,600 who have died in the conflict between the Saudi-backed government and rebels since 2015.

A disruption of water supplies could reverse recent gains in containing the spread of cholera, which reached about 900,000 suspected cases over the past year, Jamie McGoldrick, the U.N. humanitarian chief for Yemen, told The AP.

In the port city of Hodeida, caught in the blockade by a Saudi-led military coalition fighting Yemeni rebels, pediatrician Ousan al-Absi at al-Thawra hospital said three people had died of diphtheria in 24 hours.

Saudi Arabia on Thursday intercepted and destroyed a ballistic missile fired from within Yemen, its state media reported, the second such attack this month claimed by Houthi rebels, according to Agence France-Presse (AFP).

The missile targeted the southern Saudi city of Khamis Mushait, with authorities reporting no casualties, just hours after the rebels threatened to retaliate over a crippling blockade on Yemen.

The Houthi rebels this month warned that they considered "airports, ports, border crossings and areas of any importance" in Saudi Arabia and its ally the United Arab Emirates (UAE) to be legitimate targets.