Humanitarian agencies say a possible U.S. designation of Yemen's Houthi group as a foreign terrorist organisation would prevent life-saving aid to the conflict-riven country, where fears of famine are rising.

Washington sees the group, which controls northern Yemen and its most populated areas, as an extension of Iranian influence in the region. President Donald Trump's administration has been threatening to blacklist the group, sources have told Reuters, as part of its "maximum pressure" campaign against Tehran.

Over the past few weeks concern has risen among humanitarian groups that such a designation might be imminent, sources said.

Two separate sources told Reuters around a dozen American aid workers had left the country this week. Other sources said an informal note had been sent to American aid workers last week, in anticipation of potential safety concerns in Houthi-controlled areas.

U.N. spokesman Stephane Dujarric declined to comment on security issues, though he said there had been some "basic rotations of staff."

When asked about the possible Houthi designation, he said he would not comment "on something that is yet to happen," but added: "The growing risk of famine in Yemen underlines the need for us to continue to have access .... The humanitarian situation in Yemen has never been worse."

Aid agencies worry their work would be criminalised. The Houthis - also called Ansar Allah - are the de facto authority in northern Yemen and humanitarian organisations have to get their permits to carry out aid programmes, as well as work with ministries and local financial systems.

Should the U.S. government proceed, it should also issue a 'General License' that ensures humanitarian groups can still work with Houthi authorities and provide aid, said Jan Egeland, a former U.N. aid chief who now heads the Norwegian Refugee Council (NRC), which works in Yemen.

"We must be able to negotiate access for our aid and protection of civilians with all sides to all conflicts," he said.

The U.N. describes Yemen, already poor before almost six years of war, as the world's biggest humanitarian crisis, with 80% of the population reliant on aid.

This year the coronavirus pandemic, economic decline, floods, escalating armed conflict and a severe aid funding shortage have again raised the possibility of famine.

"A potential designation is coming at a time of unprecedented need ... but our ability to respond is diminishing," said the NRC's Sultana Begum from Yemen.

Aid workers also worry a designation - with the increased burden on banks' compliance mechanisms - would impact Yemenis' access to financial systems and remittances from abroad, as well as complicating imports and raising goods prices further.