Sana'a - Nine more United Nations personnel have been detained by Yemen's Houthi authorities, bringing the total number of arbitrarily held UN staff to 53 since 2021, UN spokesperson Stephane Dujarric said on Monday.

"The Secretary-General strongly condemns the continued arbitrary detentions of its personnel and its partners, as well as the ongoing unlawful seizure of UN premises and assets in areas under Houthi control," Dujarric said in a statement.

Dujarric did not specify the timing or circumstances of the latest detentions.

The group raided UN premises in Sanaa in August and detained at least 18 UN staff following an Israeli strike that killed the prime minister of the Houthi-run government and several other ministers.

Houthi officials said the UN personnel's legal immunities should not shield espionage activities, accusing the organisation of bias for condemning what they called "legal measures against spy cells" while failing to denounce the Israeli attack.

"The Secretary-General reiterates his urgent call for the immediate and unconditional release of all personnel... They must be respected and protected in accordance with applicable international law," Dujarric said.