The heads of six U.N. agencies and three international humanitarian organizations issued a joint appeal Thursday to Yemen’s Houthi rebels for the immediate release of 17 members of their staff who were recently detained along with many others also being held by the Iranian-backed group.

Their appeal was echoed by a statement from several dozen nations and the European Union ahead of a U.N. Security Council meeting on Yemen where U.N. special envoy Hans Grundberg said the Houthis were holding all those detained in the crackdown incommunicado.

The Houthis said Monday they had arrested members of an “American-Israeli spy network,” days after detaining the staffers from the U.N. and aid organizations.

Maj. Gen. Abdulhakim al-Khayewani, head of the Houthis’ intelligence agency, announced the arrests, saying the spy network had first operated out of the U.S. Embassy in the capital Sanaa. After it was closed in 2015 following the Houthi takeover of Sanaa and northern Yemen, he said, they continued “their subversive agenda under the cover of international and U.N. organizations.”

He did not say how many people were arrested. Houthi authorities issued what they purported to be videotaped confessions by 10 Yemenis, several of whom said they were recruited by the U.S. Embassy. They did not include any of the U.N. employees who were arrested. The Houthis’ claims could not be independently verified.

Samantha Power, administrator of the U.S. Agency for International Development, strongly condemned the abuse and detention of current and former USAID staff as well as U.N. and NGO employees and demanded their immediate release,

“These detentions are an affront both to diplomatic norms and to the dedication the individuals have shown to supporting the people of Yemen,” she said in a statement. “The Houthis’ attempts to spread disinformation regarding the roles of USAID, the U.S. government, the U.N., and other international organizations working to improve the lives of the Yemeni people through the use of forced and fraudulent `confessions’ is deplorable.”

The statement from the heads of the U.N. and aid organizations whose staffers are being held called their detentions “unprecedented — not only in Yemen but globally.”

They asked the Houthis to confirm the exact whereabouts of those detained and for immediate access, citing international humanitarian law which requires all parties to armed conflict to respect and protect humanitarian personnel.

“The targeting of humanitarian, human rights, and development workers in Yemen must stop,” the joint statement said. “All those detained must be immediately released.”

The statement from U.N. member nations, read by British Ambassador Barbara Woodward outside the Security Council chamber, strongly condemned the detentions since June 7, demanded the release of all those being held, and expressed grave concern at the rapid deterioration of the humanitarian situation in Yemen.

The countries expressed deep concern at the risk of delivering humanitarian aid in Yemen, and called for unimpeded access for all humanitarian workers.

The Houthis have been engaged in a civil war with Yemen’s internationally recognized government, backed by a Saudi-led coalition, since 2014, when they took control of Sanaa and most of the north.

Grundberg, the U.N. envoy who has been trying to get both sides back to the negotiating table to end the conflict, appealed not only for the release of the 13 recently detained U.N. personnel — including one from his staff — but for four other U.N. staffers being held incommunicado – two since 2021 and two since 2023.

“The United Nations is present to serve Yemenis,” he told the Security Council. “Such arbitrary detentions are not the expected signal of an actor who is seeking a mediated solution to the conflict.”

The detentions came as the Houthis have been targeting shipping throughout the Red Sea corridor over the Israel-Hamas war in the Gaza Strip.

At the same time, their administration has faced increased financial pressure, and the group has cracked down on dissent at home, including the recent sentencing of 45 people to death.

Grundberg expressed concern at the 45 death sentences, reiterating the United Nations’ opposition to the death penalty.

It’s unclear what exactly sparked the latest detentions. Former employees of the U.S. Embassy in Sanaa, which shuttered in 2015, also have been detained and held by the Houthis.