Yemen's Houthi rebels are taking hostages and committing serious abuses - including torture - against them, Human Rights Watch says.
Former detainees told the US-based group that they were beaten with iron rods, whipped, shackled to walls, caned on their feet and threatened with rape.
HRW said the hostages were used to extort money or for prisoner exchanges.
There was no immediate response from the Houthis, but they have denied past allegations of human rights abuses.
The rebels seized control of much of western Yemen in early 2015, forcing President Abdrabbuh Mansour Hadi to flee abroad and prompting a Saudi-led multinational coalition to intervene in an attempt to restore the government.
At least 6,660 civilians have been killed and 10,563 injured in fighting since then, according to the UN. Thousands more civilians have died from preventable causes, including malnutrition, disease and poor health.
The fighting and a partial blockade by the coalition have also left 22 million people in need of humanitarian aid, created the world's largest food security emergency, and led to a cholera outbreak that is thought to have affected 1.1 million people.
A report published by HRW on Tuesday said researchers interviewed 14 people who were detained by the Houthis and the relatives of two other men who were still detained or had disappeared.
A doctor said he was seized in mid-2016 by armed men from a hospital in the Red Sea port city of Hudaydah where he worked and accused of working for the anti-Houthi opposition, because he had treated a prisoner with gunshot wounds whom rebels had left on a roadside.
The doctor told HRW that guards hit him with iron rods on the soles of his feet and kicked him in the face on his first day in detention. On the second, they hung him by his cuffed arms and began removing his fingernails with pliers, he said.
He was then transferred to another facility, where he was allegedly left shackled to a wall for days, given electric shocks and beaten. He said he was released 15 months later, after his family paid 3m riyals ($11,980; £9,110) to Houthi officials.
A teacher from Hudaydah was also cited as saying he was held for three years before his family paid 10m riyals to the Houthis, while a man from the capital Sanaa was reportedly freed after his family handed over 1m riyals to mediators.
HRW warned that hostage-taking was a serious violation of the laws of war and a war crime.
"Rather than treat detainees humanely, some Houthi officials are exploiting their power to turn a profit through detention, torture, and murder," said Sarah Leah Whitson, HRW's Middle East director.
A report published last month by UN human rights experts said there was widespread arbitrary detention throughout Yemen, and ill-treatment and torture in facilities run by the Houthis, as well as those run by Yemeni pro-government and allied coalition forces.
In most cases, the experts found, detainees were not informed of the reasons for their arrest, were not charged, were denied access to lawyers or a judge and were held incommunicado for prolonged or indefinite periods.