Sky News reports that Houthis have asked British and American aid workers to leave the country within 30 days.

There were more airstrikes launched on Yemen yesterday and neither side appears to be backing down.

The full statement:

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.

This would carry more weight if it was from the US or UK because that would highlight an imminent large-scale attack. As it stands, it's not clear what would motivate the Houthi authorities to do this.