The United Nations said Yemen's Houthis detained at least 11 workers on Sunday in raids on UN premises, after rebel authorities made numerous arrests following Israel's killing of their prime minister.

The Houthi authorities have not yet commented on the reported raids, but the group has previously arrested international aid workers.

UN Secretary-General Antonio Guterres called for their "immediate and unconditional release", confirming that 11 UN personnel had been subjected to "arbitrary detentions... by the Houthi de facto authorities".

UN envoy to Yemen Hans Grundberg said the detentions, carried out in Sanaa and Hodeida, came after "the forced entry into UN premises and seizure of UN property".

The Houthis were already detaining 23 UN personnel, some since 2021 and 2023, he added. In January, the Houthi rebels detained eight UN workers.

The Houthis claimed arrests made in June 2024 included "an American-Israeli spy network" operating under the cover of humanitarian organisations – allegations emphatically rejected by the UN.

Earlier Sunday, the World Food Programme said one of its staff had been detained in the rebel-held capital Sanaa.

It said it was "urgently seeking additional information" from the Houthi authorities, who seized Sanaa in 2014 and now control large parts of Yemen.

A security source in Sanaa told AFP that seven WFP employees and three Unicef workers had been arrested Sunday after their offices were raided.

The WFP statement said the "arbitrary detention of humanitarian staff is unacceptable. The safety and security of personnel is essential to carrying out life-saving humanitarian work".

Grundberg said the arrests violated "the fundamental obligation to respect and protect their (UN personnel) safety, dignity, and ability to carry out their essential work in Yemen".

A decade of civil war has plunged Yemen into one of the world's worst humanitarian crises, with more than half of the population relying on aid.

The arrests last year prompted the UN to limit its deployments and suspend activities in some regions of the Arabian Peninsula's poorest country.

On Saturday, a Yemeni security source told AFP that Houthi authorities had arrested dozens of people in Sanaa and other areas "on suspicion of collaborating with Israel".

It came after Israel's strike on Thursday that killed the Houthis' prime minister, Ahmed Ghaleb Nasser al-Rahawi, along with other officials.

The Iran-backed group on Sunday vowed to intensify its attacks on Israel after the killing of Rahawi, the most senior Houthi official known to have died in a series of Israeli attacks during the Gaza war. (AFP)