The Iran-backed Houthi movement carried out raids on Sunday against the UN's food program (WFP) and the children's fund (UNICEF) offices in Yemen's capital Sanaa.
Eleven UN employees have been detained by the Houthi rebels and the UN's special envoy to Yemen, Swede Hans Grundberg, demands that they be released "immediately and unconditionally".
He says in a statement that they strongly condemn "the new wave of arbitrary arrests of UN personnel" as well as "intrusions into UN premises and seizure of UN property".
The raids took place in a situation of tightened security controls in Sanaa, after Israel's air force killed the Houthi movement's prime minister Ahmed al-Rahawi and several other leaders on Thursday.
The UN has long been pressured by the rebels, who have previously detained dozens of employees and collaborators of international organizations.
Now they fear that it will lead to a further escalation of the Houthi movement's conflict with both Israel and Western countries, since the group has threatened to continue its attacks on Israeli targets and merchant ships in the Red Sea.
Ever since the outbreak of war in Gaza, the Houthi movement in Yemen has attacked ships in the Red Sea to avenge Israel.
Yemen has been ravaged by a long-standing civil war and the Houthi movement has controlled large parts of the country for several years.
