Human Rights Watch on Tuesday accused Yemen's Huthi rebels of hostage-taking, torture and other serious abuses against people in their custody.
The New York-based watchdog said it had documented 16 cases of illegal imprisonment by the Iran-backed Shiite insurgents, "in large part to extort money from relatives or to exchange them for people held by opposing forces".
"Huthi officials have treated detainees brutally, often amounting to torture," HRW said, adding that former detainees described being beaten with iron rods, wooden sticks and assault rifles.
Prisoners were shackled to walls, caned and threatened with rape, it said, noting that hostage-taking "is a serious violation of the laws of war and a war crime".
"The Huthis have added profiteering to their long list of abuses and offences against the people under their control in Yemen," said HRW Middle East director Sarah Leah Whitson.
"Rather than treat detainees humanely, some Huthi officials are exploiting their power to turn a profit through detention, torture and murder."