Aid agencies were thrown into confusion Monday over the Trump administration's out-the-door decision to designate Yemen's Iranian-backed rebels as a terror organization.

Aid agencies were thrown into confusion Monday over the Trump administration's out-the-door decision to designate Yemen's Iranian-backed rebels as a terror organization, which they warned could wreck the tenuous relief system keeping millions alive in a country already near famine in the world's worst humanitarian disaster.

The designation is to take effect on President Donald Trump's last full day in office, a day before President-elect Joe Biden's inauguration on Jan. 20. Several aid groups pleaded on Monday for Biden to immediately reverse the designation. The Biden transition team has not yet expressed his intentions.

``Acting on day one cannot only be a figure of speech,'' Oxfam America's Humanitarian Policy Lead Scott Paul said. ``Lives hang in the balance.''

Six years of war between a US backed Arab coalition and the Iranian-backed Houthi rebels have been catastrophic for Yemen. Most of its 30 million people rely on international aid to survive. The UN says 13.5 million Yemenis already face acute food insecurity, a figure that could rise to 16 million by June.

Aid agencies said Monday they were struggling to figure out the implications of the designation, which would bring sanctions against the Houthis. Some were considering pulling out foreign staff. They warned that even if the US grants humanitarian exceptions as Secretary of State Mike Pompeo promised Sunday, the move could snarl aid delivery, drive away banks and further wreck an economy in which millions can't afford to feed themselves.

The Houthis rule the capital and Yemen's north where the majority of the population lives, forcing international aid groups to work with them. Agencies depend on the Houthis to deliver aid and pay salaries to Houthis to do so. Still, the rebels have been implicated in stealing aid and using aid access to extort concessions and money, as well as in a catalog of human rights abuses including rape and torture of dissidents.

Houthi officials Monday were defiant over the US designation.

``We are not fearful,'' tweeted the head of the group's Supreme Revolutionary Committee, Mohammed Ali al-Houthi. ``America is the source of terrorism. It's directly involved in killing and starving the Yemeni people.''

In Iran, the Foreign Ministry spokesman Saeed Khatibzadeh said the designation was ``doomed to failure'' and the US would eventually have to enter negotiations with the Houthis.

The US designation move is part of the Trump administration's broader effort to isolate and cripple Iran. It also shows support to its close ally, Saudi Arabia, which leads the anti-Houthi coalition in the war. Saudi Arabia has advocated the terror designation, hoping it would pressure the rebels to reach a peace deal. Past rounds of peace talks and cease-fire agreements have faltered.

The Saudi Foreign Ministry welcomed the US decision, saying that it hopes the designation would force the rebels to ``seriously'' return to negotiating table.

Maged al-Madhaji, the director of Yemen's most prominent think tank, Sanaa Center for Strategic Studies, said the designation will ``shut the doors of (Houthi) attempts to win international legitimacy.'' It will also ''paralyze their finances and drain money coming from regional allies,'' he said.

But the designation could hamper UN mediation efforts and hurt peace talks by polarizing each side's positions, said UN secretary-general spokesman Stephane Dujarric.

Yemen's war has killed more than 112,000 people and has reduced to ruins the country's infrastructure, from roads and hospitals to water and electricity networks. It began with the Houthi takeover of the north in 2014, which prompted a destructive air campaign by the Saudi-led coalition, aimed at restoring the internationally recognized government.

The Houthis receive financial and military support from Iran, and they have pelted Saudi cities with missiles and drone strikes. Their opponents say they aim to impose an Iranian-style fundamentalist rule under the group's religious and military leader, Abdel-Malek al-Houthi.

Already, the US, which is one of the largest donors to Yemen, suspended millions of dollars in aid to Houthi-controlled areas after reports of theft and looting of relief supplies. UN agencies have long complained of rebels stealing and rerouting food aid.

Announcing the decision late Sunday, Pompeo promised measures to reduce the impact on humanitarian efforts. Those include special US Treasury licenses to allow US aid to continue flowing and aid agencies to continue working.

Still, aid groups said the impact could be disastrous.

``Even with exemptions, the operation will be compromised,'' said Janti Soeripto, the president of Save the Children. She criticized the ``chaotic manner'' by which the US took the decision, which she said left agencies scrambling to figure out the implications and how to deal with them.

Delays or confusion in the license-issuing process could slow or disrupt imports of food, medicine and fuel, at a time when even slight interruptions have major consequences.

International banks are likely to retreat from any dealings with Yemen's Houthi-dominated banking system. That will cripple aid agencies and NGOs, which must use the banks to move funds, pay salaries and keep operations running. It could also hurt a main income source for individual Yemenis  remittances from relatives abroad.

``Yemen's faltering economy will be dealt a further devastating blow,'' said Mohamed Abdi, Yemen director for the Norwegian Refugee Council. He said banks, businesses and donors could ``become unwilling or unable to take on the risk of operating in Yemen.''

Once the designation is in effect, the World Health Organization will carry out contingency plans to relocate its international staffers, said its country director in Sanaa, Adham Ismail.

``It will definitely limit our capacity to bring international expertise,'' he said, as well as make it harder to get donations and deliver health services to Yemenis under Houthi control.

Peter Salisbury, Yemen expert at the International Crisis Group, said ``there is a broad consensus that if mismanaged this could have a devastating impact on millions of ordinary Yemenis.''

``What is less clear is how much of an immediate impact it will have on the Houthis, who will be the last to starve.''