UN agencies and NGOs expressed "grave concern" Saturday over the referral for criminal prosecution of a large number of their staff who have been "arbitrarily detained" by Yemen's Houthi rebels, and renewed calls for their immediate release.

The Houthis have detained dozens of staff from UN and other humanitarian organisations, most of them since June, claiming they are members of a "US-Israeli spy network," a charge the United Nations denies.

"We are extremely concerned about the reported referral to 'criminal prosecution' by the Houthi de facto authorities of a significant number of arbitrarily detained colleagues," said a statement signed by principals of affected UN entities and international NGOs.

The Houthi authorities have not issued any announcement in this regard.

The signatories of the statement included WHO chief Tedros Adhanom Ghebreyesus, UNESCO head Audrey Azoulay, UN human rights chief Volker Turk and Oxfam International executive director Amitabh Behar.

The Houthis have kidnapped and arbitrarily detained hundreds of civilians, including UN and NGO workers, since the start of Yemen's civil war in 2014, according to rights groups.

In June, the Houthis detained 13 UN personnel, including six employees of the Human Rights Office, and more than 50 NGO staff plus an embassy staff member.

The Houthis claimed they had arrested "an American-Israeli spy network" operating under the cover of humanitarian organisations -- allegations emphatically rejected by the UN Human Rights Office.

Two other UN human rights staff had already been detained since November 2021 and August 2023 respectively. They are all being held incommunicado.

In early August, the Houthis stormed the UNHCR office, forced staff to hand over the keys, and seized documents and property, before returning it later that month.

The signatories of the statement Saturday renewed their "urgent appeal for the immediate and unconditional release" of all detained staff.

The Houthis overran the capital Sanaa in 2014 and hold most of the country's main population centres, forcing the internationally recognised government to flee to Aden.

A Saudi-led coalition intervened to prop up the beleaguered government the following year.

The war in Yemen has caused hundreds of thousands of deaths and triggered one of the world's worst humanitarian crises. Fighting has significantly decreased since the negotiation of a six-month truce by the UN in April 2022.