Yemen's Houthi authorities have ordered US and British staff of the United Nations and Sanaa-based humanitarian organisations to leave the country within a month, a document and a Houthi official said on Wednesday.

The decision follows strikes by the United States and Britain, with support from other nations, against military targets of the Iran-aligned group, which has been launching attacks on commercial ships in the Red Sea that is says are linked to Israel.

The US government last week also returned the Houthis to a list of terrorist groups as Washington tries to stem attacks on international shipping in the Red Sea. The Houthis have said their attacks are in solidarity with the Palestinians as Israel bombards Gaza.

"The ministry ... would like to stress that you must inform officials and workers with US and British citizenships to prepare to leave the country within 30 days," said a letter sent by the Houthi foreign ministry to the UN's acting humanitarian coordinator in Yemen, Peter Hawkins.

The letter also ordered foreign organisations to not hire American and British citizens for Yemen's operations.

Houthi top negotiator Mohammed Abdulsalam confirmed the letter's authenticity to Reuters.

The office of Hawkins, who is himself a British national, did not respond to a request for comment.

The US embassy said in a statement it was aware of reports about the letter but "cannot speak on behalf of the UN or humanitarian organizations in Yemen as to what they may have received from Houthi 'authorities'".

The British embassy said staff had not yet been told to leave and the mission was in close contact with the UN on the issue.

"The UN provide vital assistance to the Yemeni people ... via the very sea routes that the Houthis are jeopardising," the British mission in Yemen said in a statement. "Nothing should be done that hinders their ability to deliver," it added.

The Houthi movement controls much of Yemen after nearly a decade of war against a US-backed and Saudi-led coalition. The war has shifted to a "no-war, no-peace" stalemate as the fighting has largely stopped, but both parties have failed to renew formally a UN-brokered ceasefire.

US and British warplanes, ships and submarines have launched dozens of air strikes across Yemen in retaliation for Houthi attacks as container vessels have been forced to divert from the Red Sea, the fastest freight route from Asia to Europe.

US and British forces on Tuesday targeted a Houthi underground storage site as well as missile and surveillance capabilities, the Pentagon said.