UN High Commissioner for Human Rights Volker Türk announced the death of Mohammad El Shami on Saturday, a Yemeni national who worked for the UN Human Rights Office, describing it as "utterly devastating".
El Shami died more than a month ago, but his family was only recently informed of his death, according to information provided to the UN by his relatives.
"Mohammad's family has not received any explanation of the cause or circumstances of his death - they deserve to know the truth," Türk said on X.
He demanded full transparency from the Houthi authorities and an urgent, independent investigation, including access to medical and detention records.
UN Secretary General António Guterres was appalled and condemned the death, his spokesman Stéphane Dujarric said in a statement.
"The circumstances surrounding this appalling tragedy remain unclear," Dujarric said.
Since his detention, Houthi authorities had provided neither information about his well-being nor proof of life, despite repeated requests from the UN, he said.
It was the second death of a UN staff member detained by Houthi authorities, following the death of a World Food Programme staff member in February 2025.
"It is a painful reminder of the grave risks faced by all UN and humanitarian personnel, who remain arbitrarily detained - some for years. Their continued detention is unacceptable and causes immense suffering to them and their families," Dujarric said.
"The Secretary-General recalls that all persons deprived of their liberty must be treated humanely and with respect for their inherent dignity and human rights. The authorities exercising custody bear full responsibility for their protection, safety, access to medical care, and physical and psychological well-being."
Guterres remained "gravely concerned for the safety and physical and psychological well-being" of the 70 United Nations personnel, five former UN staff members, and personnel from humanitarian and civil society organisations who remain detained.
"He reiterates his call for their immediate and unconditional release," Dujarric said.
A Yemeni government minister said earlier that El Shami had been tortured to death in a Houthi prison, Saudi broadcaster Al Arabiya reported.
The allegation has not been independently verified, and Türk's statement did not specify the cause of death.
Türk also expressed concern for other UN personnel still held by the Houthis, including seven employees of his office, and called for their immediate release.
The Iranian-backed rebels typically justify such detentions by accusing those held of spying for the United States or Israel.
Yemen's civil war began in 2014, when the Houthis seized the capital Sana'a and large parts of the north. A Saudi-led coalition launched airstrikes against Houthi targets in 2015.
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