Yemen's Iran-backed Houthi rebels are an "affront" to the international community and to the Yemeni people, US Special Envoy for Yemen Tim Lenderking said on Friday.
Mr Lenderking's comments came one year since the rebels seized the headquarters of the US embassy in the capital Sanaa and detained dozens of former staff, many of whom were later released.
The compound housed the US embassy until 2015, when the US suspended operations there because of the continuing war between the Houthis and the internationally recognised government, with America moving its ambassador and key staff to Saudi Arabia.
The rebel's actions are an "affront to the entire international community, but more importantly to Yemenis themselves", Mr Lenderking said.
"One year later, 12 US and UN employees, all Yemeni citizens, remain in Houthi detention in Sanaa without any justification and little to no contact with their families," his statement read.
One of those detained, Adbulhameed Al Ajami, died in custody.
"How much more suffering will the Houthis inflict on Yemeni families? We stand by our staff and will continue efforts to secure their release," Mr Lenderking said.
UN agencies confirmed late last year that the Houthis had also arrested two of their employees in Sanaa in early November.
The UN's cultural agency, Unesco, and the UN human rights office said no legal grounds were given for their detention.
The UN said the civil war has created the world’s greatest humanitarian crisis.
Mr Lenderking's comments came shortly after the Houthis rejected a renewal of a nationwide truce that has brought much-needed calm to the war.
The UN-led ceasefire came into effect at 7pm on April 2 and was extended for two months twice.
However, Yemen's parties failed to reach agreement for a further six months.
The conflict in Yemen started eight years ago after the takeover of Sanaa by the Houthis, prompting a Saudi-led coalition to intervene on behalf of the internationally recognised government.
