The decision follows the United States and Britain, with support from other nations, striking 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 U.S. government last week also returned the group 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 strikes Gaza.
"The ministry ... would like to stress that you must inform officials and workers with U.S. and British citizenships to prepare to leave the country within 30 days," said a letter sent by the Houthi foreign ministry to the U.N.'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 U.S. and British embassies in Yemen did not immediately respond to a request for comment.
The Houthi movement controls much of Yemen after nearly a decade of war against a U.S.-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 U.N.-brokered ceasefire.
U.S. 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.
U.S. and British forces on Tuesday targeted a Houthi underground storage site as well as missile and surveillance capabilities, the Pentagon said.
(Additional reporting and writing by Aziz El Yaakoubi; Editing by Alison Williams)
By Mohammed Alghobari and Riyam Mukhashaf
