Impoverished Yemen would be at risk of a devastating famine should the outgoing Trump administration push ahead with a move to designate the country’s largest rebel group as a terrorist organisation, the UN’s aid chief warned.

Mark Lowcock, UN under-secretary-general for humanitarian affairs, said food imports to the impoverished Arab country plummeted by a quarter in November after US media quoted officials as saying that President Donald Trump planned to label the Iran-aligned Houthi movement as a foreign terrorist organisation before his term ended. Food import numbers had not improved in December, Mr Lowcock added. Aid groups say that Yemen, where a multi-sided war has raged for nearly six years, faces the world’s worst famine in decades.

“There’s a chilling effect of the [terrorist] designation . . . that risks undermining imports of food and becomes the final straw that tips the country into not just a small famine, but a large one,” Mr Lowcock told the Financial Times in an interview. “The question is whether it’s going to become a large famine or a truly huge famine on a scale the likes of which the world has not seen since 1m people lost their lives in Ethiopia in the 1980s.”

Yemeni traders feared being “caught in the implications” of any move to designate the Houthis a terrorist organisation, he said. The traders worked with “commercial partners and those people will be even more nervous,” he added.

The Houthi movement controls Yemen’s populous north, including Sana’a, the capital, and Hodeidah, a port city through which about two-thirds of the import-dependent nation’s goods are shipped. If the movement was labelled a terrorist group, any individual or organisation transacting with the Houthis would risk US sanctions.

Washington has not commented publicly on any plans to designate the Houthis a terrorist organisation, but US media quoted administration officials last month that Mr Trump was considering the move before he left office on January 20.

The Trump administration, and its allies in Saudi Arabia and the United Arab Emirates, consider the Houthis to be an Iranian proxy. They accuse Tehran of supplying arms to the group, including missiles and drones used to attack Saudi Arabia.

The Houthis, an Islamist movement, have controlled Sana’a since taking over the capital and ousting the government in 2015. Saudi Arabia then intervened militarily against the Houthis as Yemen descended into war, which has triggered what the UN describes as the world’s worst man-made humanitarian crisis.

The UN has been providing food aid for about 13m Yemenis at risk of starvation — about half the population — but a decline in donor funding, particularly from Saudi Arabia, the UAE and Kuwait, has meant it is now only able to feed 9m, Mr Lowcock said. The 4m people no longer receiving assistance were “basically in the process of being starved”.

A western diplomat said US officials he had spoken to recognised that designating the Houthis a terrorist organisation would exacerbate the dire situation.

There is also the perception that the Trump administration has considered the move to make it more complicated for president-elect Joe Biden to deal with crises in the Middle East, the diplomat said.

Mr Lowcock said that while US officials have said the Trump administration would exempt humanitarian organisations from any sanctions, the vast majority of food consumed in Yemen was imported by commercial traders.

UN agencies provide food aid directly to some Yemenis and gives others cash transfers and vouchers that enable them to buy goods on the market. The concerns is that if food imports dropped sharply, more people would become dependent on direct aid at a time when agencies had less funding.

“The situation was on a deteriorating trend before because of ongoing violence and because we have not received the pledges we need for humanitarian assistance,” Mr Lowcock said. “What importers are saying to us is they are worried about their ability to continue importing grain if this designation of Houthis goes ahead, and that’s my main concern.