Pro-Russian insurgents in Ukraine are committing atrocities against civilians, Human Rights Watch (HRW) has warned.
In its latest report, the rights group shows evidence of abductions and torture of Ukrainian civilians by the Russia-backed rebels.
Since last April, armed fighters supporting the self-proclaimed republics of Donetsk and Luhanks, in eastern Ukraine, have captured hundreds of civilians, including pro-Ukrainian activists and journalists.
"Pro-Russian insurgents are regularly committing horrendous crimes," said Hugh Williamson, Europe and Central Asia director at HRW. "There are solid grounds to be seriously concerned about the safety and well-being of anyone held by insurgent forces in eastern Ukraine."
HRW documented the abduction of 20 people, kidnapped by the rebels. At least 12 of the victims were also beaten and tortured.
Pro-Ukrainian activist Olha Klimenko was detained for six days, tortured and used for forced labour.
"They [interrogators] asked me if I was involved with Euromaidan," he told HRW. " They also wanted to know if I worked for one of the election commissions [for the May 25 Ukrainian presidential elections].
"Those who stood behind me beat me for giving snide replies or thinking too much before answering their questions. They punched me on the head, on the liver, in the solar plexus. One of them put his gun to my head and pulled the trigger. The gun wasn't loaded, but I did not know that...Their interrogations were all about breaking you."
Another pro-Ukrainian activist, Anna Guz, was detained with her partner for five days. Both were tortured.
"He [the torturer] was yelling non-stop and waving his knife close to my face...."Guz said. "He was hitting me on the knees with handcuffs, then poked me with his knife on one knee, pushed the knife in by half an inch, and turned it.
"He did the same to my other knee. He would run out and then come back and torture me again. He threatened me with gang rape...They said that if I wanted to live I had to trick other activists and journalists into coming to them."
HRW urged the rebels to stop committing atrocities against civilians. The group also expressed concern for cases of extra-judicial executions.
"Self-proclaimed authorities in eastern Ukraine should immediately free anyone held arbitrarily," Williamson said. "Russia should use its influence with insurgent forces in eastern Ukraine to stop these blatant violations and ensure that those responsible are brought to justice."
Ukraine accused Russia on Friday of launching a full-scale invasion.
Although Russia denied, Nato confirmed the allegation and, citing satellite images, said hundreds of troops are entering southern Ukraine.