The heads of six United Nations agencies and three international NGOs issued a joint call on Thursday for the release of their staff detained by Yemen’s Iran-backed Houthi rebels. They said the detentions were unprecedented and insisted the targeting of aid and development workers in Yemen must stop. “We, the principals of the affected UN entities and international non-governmental organisations, call for the immediate and unconditional release of all personnel held in Yemen by the Houthi de facto authorities,” the joint statement said. “We are extremely concerned about the Houthi de facto aut…