Iranian-backed Houthi rebels detained two dozen UN employees on Sunday, a day after they raided another UN facility in the capital Sanaa, a UN official said.

Jean Alam, a spokesman for the UN resident coordinator for Yemen, told The Associated Press that the UN staffers were detained inside the facility in Sanaa’s southwestern neighborhood of Hada.

He said those detained Sunday include five Yemenis and 15 international staff. He said the rebels released another 11 UN staffers after questioning.

He said the United Nations was in contact with the Houthis and other parties to “resolve this serious situation as swiftly as possible, end the detention of all personnel, and restore full control over its facilities in Sanaa.”

A second UN official, speaking on condition of anonymity to discuss the raid, said the rebels confiscated all communications equipment from the facility, including phones, servers, and computers.

The official said the detained employees belong to multiple UN agencies, including the World Food Program, UNICEF, and the Office for the Coordination of Humanitarian Affairs.

The Houthis have launched a long-running crackdown against the UN and other international organizations working in rebel-held areas in Yemen, including Sanaa, the coastal city of Hodeida, and the rebel stronghold in Sadaa province in northern Yemen.

Dozens of people, including over 50 UN staffers, have been detained so far. A World Food Program worker died in detention earlier this year in Sadaa.

The rebels have repeatedly alleged without evidence that the detained UN staffers and those working with other international groups and foreign embassies were spies. The UN fiercely denied the accusations.

The crackdown forced the UN to suspend its operations in Saada province in northern Yemen following the detention of eight staffers in January. The UN also relocated its top humanitarian coordinator in Yemen from Sanaa to the coastal city of Aden, which serves as the seat for the internationally recognized government.