Rights group says arbitrary detention, torture, and enforced disappearances have gone unpunished in Houthi-held areas.
Human Rights Watch (HRW) has accused Yemen's Houthi rebels of torture, hostage-taking, and other serious abuses against people in their custody.
The New York-based watchdog said in a report released on Tuesday that it had documented 16 cases of illegal imprisonment by the rebels, "in large part to extort money from relatives or to exchange them for people held by opposing forces".
"Houthi 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.
It said prisoners were shackled to walls, caned, and threatened with rape, noting that hostage-taking "is a serious violation of the laws of war and a war crime".
Former detainees described terrible conditions in Houthi custody: poor hygiene, limited access to toilets, which caused some to defecate on themselves; and lack of food and healthcare.
"Some Houthi officials are exploiting their power to turn a profit through detention, torture, and murder," HRW's Middle East director Sarah Leah Whitson said.
The Association of Mothers of Abductees, a group run by Yemeni women who advocate for their arrested or disappeared civilian relatives, sent HRW accounts from 10 cases in which Houthi officials demanded money as a condition for release.
According to the association, which holds regular public protests, nine families paid. Houthi officials released only three of the men, including one in a prisoner exchange for Houthi fighters.
UN investigators reported last month that all sides in the conflict may have committed war crimes, pointing to widespread arbitrary detention, rape, torture, and the recruitment of children.
A report obtained by Al Jazeera last month alleged that the UAE had set up a network of clandestine prisons across southern Yemen. The report described scenes of sexual abuse by Emirati army personnel and their Yemeni allies.
Electric cables were used alongside wooden bats and steel poles during the interrogation sessions.
In some instances, the detainees are described as having been deprived of sleep and confined to narrow spaces with poor hygienic conditions and limited air ventilation.
HRW called on the UN Human Rights Council to renew the mandate for a group of experts on Yemen to investigate and identify all parties responsible for abuses.
"The United Arab Emirates, UAE proxies, and Yemeni government forces have also arbitrarily detained, tortured and forcibly disappeared scores of people in the Yemeni conflict," it said.