The United Nations said on Tuesday that one of its staff members had died in detention in northern Yemen, where the U.N. has repeatedly called for the Houthi movement to free detained U.N. employees.

The total employees arrested since 2021 has risen to 24, a U.N. spokesman said. The Iran-backed Houthis have held them on murky charges of espionage and foreign intervention.

The war in Yemen has killed more than 150,000 people, including fighters and civilians. The Iranian-backed Houthis have been fighting Yemen’s internationally recognized government, which is backed by a Saudi-led coalition, since 2014, when they descended from their stronghold in Saada and took control of Sanaa and most of the north.