The raid took place on Saturday, when individuals belonging to the Huthi movement's security forces "without permission" entered the premises, says the UN office in a statement, and adds that the staff is "unharmed".
Among the detained are five Yemeni citizens and 15 employees from other countries.
The storming follows a raid on August 31, when the movement detained at least eleven people from UN premises in Sanaa. At that time, a high-ranking source within the Iran-backed Huthi movement claimed in an interview with AFP that the UN employees were suspected of spying on behalf of the US and Israel.
Stéphane Dujarric, spokesperson for UN Secretary-General António Guterres, describes the arrests in a statement as "dangerous and unacceptable" and says that the UN "will continue to demand an end to the arbitrary detentions".
Yemen is ravaged by a long-standing civil war and the Huthi movement has controlled large parts of the country for several years.
