The Houthi foreign affairs ministry in Yemen wrote a letter telling British and US aid workers to leave the country within the next 30 days, according to reports.
It was sent to the country’s United Nations coordinator by the Houthi-controlled foreign affairs ministry.
The letter, reported by Sky News on Thursday, has also said aid organisations should not hire any employees with British or American citizenship.
It says: "The Ministry of Foreign Affairs of the Republic of Yemen extends its best regards to the office of the United Nations Resident Coordinator in Sanaa, and through them to all offices of humanitarian organisations working in the Republic of Yemen.
"The ministry would like to emphasise the necessity of informing all officials and workers who hold American and British citizenship of their preparation to leave the country within a maximum period of 30 days from the date of this determination so they will be ready to leave immediately upon the expiration of the period referred to, and the ministry will be sure to inform you via a ministerial letter 24 hours before departure.
"The ministry also calls upon not to recruit any employees with dual nationalities from those countries mentioned above during this period.
"The Ministry of Foreign Affairs of the Republic of Yemen takes this opportunity to express appreciation and respect to the office of the United Nations Resident Coordinator."
It comes after the UK and US launched a fresh set of strikes in Yemen on Monday.
Defence Secretary Grant Shapps said the latest round of attacks were in "self-defence" and would "deal another blow" to the Iranian-backed militants.
It follows seven rounds of airstrikes on Houthi military sites by the US in recent days and comes after the UK took part in an initial joint strike operation earlier this month against the Houthis.
Since then, ships have continued to be targeted along the vital Red Sea and Gulf of Aden trade routes, with the US military confirming the latest strikes were against eight targets.
Mr Shapps said: "Dangerous Houthi attacks on shipping in the Red Sea have continued to threaten the lives of sailors and disrupt shipping at an intolerable cost to the global economy. Along with our US partners, we have conducted a further round of strikes in self-defence.
"Aimed at degrading Houthi capabilities, this action will deal another blow to their limited stockpiles and ability to threaten global trade. Alongside our ongoing diplomatic efforts, we will continue to support regional stability across the Middle East, working hand in hand with our like-minded partners."