Former Ukrainian captives say they were subjected to torture, including frequent beatings and electric shocks, while in custody at a detention facility in south-western Russia, in what would be serious violations of international humanitarian law.

In interviews with the BBC, a dozen ex-detainees released in prisoner exchanges alleged physical and psychological abuse by Russian officers and guards at the Pre-Trial Detention Facility Number Two, in the city of Taganrog.

The testimonies, gathered during a weeks-long investigation, describe a consistent pattern of extreme violence and ill-treatment at the facility, one of the locations where Ukrainian prisoners of war have been held in Russia.

Their allegations include:

The BBC has been unable to independently verify the claims, but details of the accounts were shared with human rights groups and, when possible, corroborated by other detainees.

The Russian government has not allowed any outside bodies, including the United Nations and the International Committee of the Red Cross, to visit the facility which before the war was used exclusively to hold Russian prisoners.

Russia’s defence ministry did not respond to several requests to comment on the allegations. It has previously denied torturing or mistreating captives.

The prisoner swaps between Ukraine and Russia are a rare diplomatic achievement in the war and more than 2,500 Ukrainians have been released since the start of the conflict. Up to 10,000 captives are believed to remain in Russian custody, according to human rights groups.

Dmytro Lubinets, Ukraine’s human rights ombudsman and one of the officials involved in exchange negotiations with Moscow, said nine in every 10 former detainees claimed they had been tortured while in Russian captivity. “This is the biggest challenge for me now: how to protect our people on the Russian side,” Lubinets said. “Nobody knows how we can do it.”

Last September, Artem Seredniak, a senior lieutenant, had already been in Russian captivity for four months when he and about 50 other Ukrainians were transferred to Pre-Trial Detention Facility Number Two. They travelled in the back of a truck for hours, without knowing where they were going, blindfolded and tied to each other by their arms, like a “human centipede”, Seredniak told me.

On their arrival in Taganrog, he recalled, an officer greeted them: “Hello boys. Do you know where you are? You’ll rot here until the end of your lives.” The captives remained silent. They were escorted inside the building, Seredniak said, had their fingerprints taken and clothes removed, were shaven and forced to shower.

At every step, guards at the facility, who carried black batons and metal bars, beat them in the legs, arms, or “anywhere they wanted”, Seredniak said. “It’s what they call ‘reception’.”

Before his capture, Seredniak, who is 27, headed a sniper platoon at the Azov Regiment, the main military force in Mariupol. This, he said, made him a key target for the prison staff. Seredniak said he was separated from the others and, dressed only in his underwear, brought to a room to be interrogated for the first time. He was then pushed to the floor, he said, with his head facing down.

The guards asked him about his role in the army and the tasks he had carried out. With an electric stun weapon, they gave him shocks, Seredniak said, in his back, groin and neck.

“That’s how they worked on everybody,” he said. “They hammered you like a nail.”

In May last year, as Mariupol was under a Russian siege, the Ukrainian authorities ordered hundreds of soldiers holed up in the city’s Azovstal steelworks to surrender. Seredniak was among the last to be evacuated. He was first taken to a facility in Olenivka, a village in Donetsk, and, months later, sent to the prison in Taganrog, in the Russian border region of Rostov, about 120km (74 miles) east of Mariupol.

There, he told me, the captives were inspected twice a day, and anything appeared to be a motive for guards to abuse them. “They might not have liked how you left the cell, or you weren’t quick to get out, or your arms were too low or your head was too high.”

In one of those checks, Seredniak was asked whether he had a girlfriend. He said he did, and recalled a guard telling him: “Give us her Instagram. We’ll take a picture of you and send it to her.” He lied, not wanting to expose her, and said she did not have an account. He was then beaten, he said, and brought to a room in the prison’s basement, where he met a twenty-something Ukrainian fighter. Seredniak told me the man was curled, holding his hands, apparently in pain, and said officers had inserted needles under his fingernails.

As the days progressed, Seredniak noticed that the prison guards were particularly brutal with those who belonged to the Azov Regiment, the former militia in Mariupol that once had links to the far right. Russia’s president, Vladimir Putin, has said, among other things, that his war is an effort to “de-Nazify” Ukraine – a country led by a Jewish president, Volodymyr Zelensky – and Russian authorities often cite the unit to justify the invasion.

Seredniak said that, in his interrogations, he was accused of looting Mariupol and of personally telling his forces to kill civilians in the city, the site of one of the most deadly battles in the war so far. Seredniak, who speaks fast with a loud, determined voice, denied the claims, but it did not seem to matter. “Until you said what they were interested in, and in the way they wanted to hear,” he told me, “they wouldn’t stop beating you.”

Once, Seredniak said, an officer used a wooden chair to hit him, and “he beat me so much that it broke in parts”. On another day, he said, he was asked whether he could sing the “Azov anthem”. He did not know of any Azov anthem, and assumed the guards meant the Prayer of the Ukrainian Nationalist, a 20th-Century oath usually read aloud by soldiers before being sent into combat. Seredniak reluctantly recited it, conscious of how the guards could react.

They punched him several times, he said. He fell, hitting his head against a wall, causing a cut near his eyebrow. He lay on the floor, while the beatings continued, he said, all over his body.

“When I finally got up,” Seredniak recalled, “they told me: ‘We hope we beat that out of you’.”

Some of the prison staff seemed to have been heavily influenced by President Putin’s “de-Nazification” narrative. For the detainees, this was apparent in how the guards demonstrated a particular interest in anything that could, in their view, be interpreted as being pro-Nazi. The captives were not allowed to have any personal items, so their tattoos inevitably drew the officers’ attention. This reminded me of similar allegations I heard while investigating Russia’s filtration camps in occupied areas of Ukraine last year.

Serhii Rotchuk, a 34-year-old senior sergeant at the regiment, also left Azovstal in the final convoys, and was taken to Taganrog a week after Seredniak. He said the guards, at first, “looked for swastikas or things like that”. But, in reality, he said, “if you had any tattoo, you were seen as a bad guy”. Rotchuk, who is a doctor, has tattoos on both legs, arms and chest. Weeks ago, when we met in Kyiv, he lifted his T-shirt to show me a raven that covered part of his chest and the symbol of an infantry platoon on his left bicep; he also had an emblem of the Jedi Order from Star Wars on his left thigh.

“Did these tattoos cause you any trouble?” I asked him. “Many times,” Rotchuk replied. “They would say: ‘What’s this? Oh, I’ll beat you for that’.” Seredniak, who has no tattoos, said some fighters who had tattooed nationalist symbols, like the Ukrainian flag or the gold trident, were frequently targeted. “They hated us for being Ukrainian,” he told me.

In March, a report by the Office of the United Nations High Commissioner for Human Rights (OHCHR) said Russia had “failed to ensure the humane treatment” of prisoners, with “strong patterns of violations”.

Kris Janowski, a spokesman for the office, said there was a “long list of bad things that have been done” to the detainees at the facility in Taganrog. The fact that a prison was being used to hold captives was, in itself, a breach of international humanitarian law, he said, as they should be kept in specially designated places. Ukraine also faced some accusations of mistreatment of detainees, according to the March report; but, overall, they were “treated in better fashion”.

Rotchuk said the captives “lived in permanent stress” in Taganrog. He recalled meeting a man, also a doctor, who had falsely admitted to removing the testicles of a Russian prisoner, desperate to put an end to the violence. “He said: ‘OK, just leave me alone, I will sign the confession.’ The officers then intimidated the other medics, saying: ‘Ah, you helped him.’”

Guards gave Rotchuk electric shocks, he said, but he resisted. Rotchuk told me he was sent to solitary confinement for two months as punishment. The beatings happened almost every day; sometimes, several times a day, he said.

Rotchuk remembered one officer who appeared to take pleasure in kicking him in the chest, which left him with a persistent pain. He complained, but was given no help. “I had to tell myself: ‘Dude, stay strong, you can’t control the situation, so you need to accept it,’” Rotchuk recalled.

Not everyone had the same resilience, though. Seredniak said a fellow Azov fighter, in his late 20s, broke a small mirror that hung above his cell’s sink, and used a shard to slice his throat. The man was rescued by other captives, who stopped the bleeding with their hands. Days later, Seredniak said, the prison staff removed the mirrors from all cells.

Russian doctors, Seredniak said, would occasionally visit the detainees, but “didn’t necessarily help” them. He described the food portions they were given as limited; sometimes, he said, they were “so small, that if I ate 300-400 calories a day, I was lucky”.

Seredniak, who is 1.86m (6ft1in) tall, said his weight dropped to about 60kg (9st 6lb) while he was there, from his usual 80kg. “Every time I got up,” he said, “I felt dizzy. My eyes darkened, I couldn’t make any fast movements.” He believed this was deliberate: weakened, the captives would not put up any resistance.

Iryna Stohnii, a 36-year-old senior combat medic at the 56th Brigade, described the detainees as “constantly malnourished”. “They didn’t feed us,” she said. “They didn’t even let us go outside… We could only see the sky through bars in the windows.” Stohnii said the guards, in their twice-a-day inspections, forced her and other women to move in a stress position, with arms behind their backs and head to knees, and that some “dragged us by the hair”. Other female captives told me women would be ordered to strip naked in front of male staff who, sometimes, made disparaging comments about their bodies.

One day, Stohnii said, a guard accused her of torturing pro-Russian soldiers in captivity and twisted her arms with so much force that “he almost broke them”. A couple of times in our interview she cried. “Only devils live and work” at Taganrog, she said. After her release, Stohnii underwent surgery to remove adhesions – bands of scar tissue between organs that can be caused by trauma – which had developed in her kidney and bladder. “Apart from rape,” Stohnii told me, “they did everything with us”.

Denys Haiduk, a military surgeon, said guards forced him and the other captives to run with their heads down while under blows during their “reception”, with detainees being hit even after they were on the ground, unable to stand up. Haiduk, who is 29, had helped the wounded at Azovstal and, in his interrogation, he was accused of amputating and castrating Russians in captivity, he told me. He denied it, saying that only Ukrainian fighters had been brought to him.

As he recalled what had happened, I could sense the anger in his voice. Haiduk was pushed to the floor, and given electric shocks with a stun weapon until, he said, the battery ran out. Other captives said guards also used a military phone to give them shocks by connecting its wires to their bodies. “You’re convulsing,” Haiduk said. “If you lift your head up, they start beating you. And that circle never ends.”

Taganrog is also used as a transfer point and, to his surprise, Haiduk was only held there for two days, before his release in a prisoner exchange. As he left, the officers tried to force him to sign a document, declaring that any injury to his body had been accidental. Haiduk refused. He said guards beat and kicked him, and he heard a crack.

Haiduk struggled to breathe, he recalled, and fell into the mattress he was holding. Later, after returning to Ukraine, he was diagnosed with three broken ribs as well as a cardiac contusion – a bruise to the heart muscle caused by trauma.

I asked him why he believed the guards were treating the Ukrainian detainees that way. “Because they can,” he said. “You’re a captive, and they abuse you.” When I asked Seredniak the same question, he gave me a more practical answer: “They beat you to get some information. And then say: ‘It’s to make sure you don’t go back and fight after the swap.’”

Lubinets, the Ukrainian ombudsman, said Russian authorities had created a “system of torture” for Ukrainian captives, typically in detention centres, in Russia and in occupied areas of Ukraine. Ukraine has opened up its facilities to experts; Russia, however, has restricted visits to only some locations. Janowski, from the OHCHR, said Moscow had repeatedly rejected the UN’s requests for access without giving “any legitimate reasons”. With most places closed to outside observers, Lubinets said, “Russian soldiers can do anything with Ukrainian prisoners”.

During his “reception”, Artem Dyblenko, a 40-year-old sergeant major at the 36th Marine Brigade, overheard the guards talking about playing football with the captives. He was intrigued. “What I didn’t know was that we would be the ball,” Dyblenko said. Blindfolded, he was ordered to run, he said, and fell. “There were constant kicks. You did feel like a football.”

Dyblenko told me that, in September, one of his cellmates suffered a heart attack, which he attributed to the constant physical abuse. No-one came to treat him, according to Dyblenko, and the man died, aged 53. Three weeks later, Dyblenko was included in a swap and, in Ukraine, reported the case to the authorities. The body, he said, was returned at the end of last year. “[His son] was given pictures of it,” Dyblenko said, “it was horrifying”. Ukraine acknowledged that bodies were exchanged in December, without giving details of the victims’ identities, or how and where they had died. The man’s son said he was waiting for the result of a DNA test and did not want to comment.

The Media Initiative for Human Rights, a Ukrainian organisation, recorded allegations of at least three deaths at the Taganrog prison, apparently because of torture and lack of food and health care. Mariia Klymyk, one of the group’s investigators, said this was “one the worst places for Ukrainian detainees in Russia”.

She heard accounts of men being taken to interrogation and asked whether they had any children. “If someone says they don’t, they are beaten in the genitals,” Klymyk said, “while the guard says: ‘For prevention of procreation.’” And some Ukrainian soldiers had been put on trial, she said, with the apparent false confessions they had given in custody used as evidence against them.

After almost 12 months in captivity, seven of them in Taganrog, Seredniak was released in a prisoner swap on 6 May, alongside 44 other Ukrainian fighters. He said the date would be celebrated as if it were his second birthday. The same exchange included Serhii Rotchuk, the doctor, who later discovered he had a fracture in his sternum – the breastbone – a condition associated with significant chest trauma, which he blamed on the abuse he had suffered.

I visited Seredniak four weeks after his return, at a flat in a residential compound on Kyiv’s left bank, between his physical and mental rehabilitation sessions. Doctors had diagnosed him with a broken rib and cysts in the liver and kidney which, they said, were probably caused by the beatings. Seredniak had already regained some of the weight he had lost but still suffered lower back pain and, sometimes, struggled to walk.

On my phone, he watched for the first time a video of his swap, which had been published by the Ukrainian government. The captives were filmed shouting “Slava Ukraini!”, or “Glory to Ukraine”, and being welcomed by a cheering crowd. Pointing at a smiling man, Seredniak said: “This is me!” I could not recognise him. “I was pale, skinny, with no access to sunlight,” he told me. “We were like bats, living in half-light.”

Additional reporting by Daria Sipigina and Lee Durant. Photos by Lee Durant