The United Nations said Monday it suspended its humanitarian operations in the stronghold of Yemen’s Houthi rebels after they detained eight more U.N. staffers, affecting the global response to one of the world’s worst humanitarian disasters.

The decision to pause all operations and programmes in northern Saada came after eight UN staff were detained by the Iran-backed Houthi rebels.

A World Food Programme (WFP) worker has died while in detention in Houthi-controlled northern Yemen, according to the UN agency. The WFP said the man was one of seven Yemeni staff "arbitrarily detained" by local authorities since 23 January. It is unclear when or how he died.