A Ukrainian armed-forces tank is stationed outside a building in the flash-point eastern town of Avdiivka, just north of the pro-Russian rebels' de facto capital of Donetsk, in this photo from February. (Alexey Filippov/AFP/Getty Images)

On a warm afternoon in Ukraine’s breakaway east, as the front line rumbled with only occasional shellfire, Stanislav Aseev, a 27-year-old undercover journalist, was heading home to Donetsk.

Many of his colleagues had long since fled the battle-scarred industrial city — a separatist stronghold where freedom of ­expression is harshly repressed. Despite this, during three years of war, Aseev remained, publishing stories under a pseudonym to avoid repercussions from the city’s new masters. Outside working hours, jazz, jogging and philosophy offered him relief from the conflict.

On June 2, as he approached Donetsk, Aseev contacted his mother and promised to visit the next day. He never arrived.

After repeated phone calls went unanswered, worried friends and relatives headed to Aseev’s apartment. His front door had been smashed open, and his possessions — including a work laptop — had been seized. Authorities in the self-proclaimed Donetsk People’s Republic (DNR) would give them no information.

It has since emerged that DNR security agents are holding Aseev incommunicado in an ­unspecified facility. His friends worry that he has been subjected to beatings and torture. That is not an unreasonable concern — according to human rights investigators, it has become standard practice for anyone in detention in Donetsk.

A Ukrainian serviceman investigates a damaged building near the front line in the Ukrainian village of Olginka earlier this month. (Sergey Vaganov/European Pressphoto Agency)

“No one can guarantee that he will survive,” said Egor Firsov, a former member of parliament and a university friend of Aseev’s who is campaigning for the journalist’s release.

Aseev has joined the hundreds of people — or possibly 1,000 or more — who are missing or held as prisoners of war
in eastern Ukraine. Although front-line hostilities have reached a simmering deadlock, a dirty war persists in the wider, lawless region. Civilians attempt to survive on contested ground, pinned between marauding forces accused of pillage, violent intimidation, sexual abuse, torture and even summary execution.

The United Nations has documented accounts of such war crimes, recording arbitrary detention and enforced disappearances across the conflict zone, particularly in territory controlled by Russian-backed separatists. Denied access to relatives or legal counsel, captives are kept in secret prisons, basements and other dire, improvised detention areas. The exact number of missing people is unknown, but the International Committee of the Red Cross says it could be up to 2,000.

“There are many sick people walking around with guns these days,” one resident of a front-line village told U.N. investigators. “If they want to kill us, they will just come. Who can protect us from this?”

A new report by the Office of the U.N. High Commissioner for Human Rights suggests both forces are carrying out extrajudicial executions. In March, the body of a missing man was found near a government-held front-line town. An officer from the SBU — Ukraine’s successor agency to the KGB — was subsequently arrested and released on bail.

Elsewhere, Ukrainian soldiers allegedly looted a house, set it on fire, shot the occupant and buried the body in a nearby forest.

Last October, in rebel-held Horlivka, two civilians were shot dead, allegedly by pro-Russian gunmen. When the corpses were found, separatist authorities are said to have prevented relatives from seeing and identifying the bodies.

One of the most high-profile victims of the wave of clandestine, wartime detentions is Igor Kozlovsky, an authoritative theologian with expertise in world faiths, martial arts and yoga. He is the former regional head of religious affairs and co-founder of an academy and spiritual center. As a university professor, he taught philosophy to Aseev — the now-imprisoned journalist — and held this gifted student in high regard.

“When he lectured, it was like watching Elvis,” said his 39-year-old nephew, Denis Kozlovsky. “He was so charismatic, so exciting — an international man but also a patriot with a deep love of Ukrainian traditions.”

When separatists moved to take power in Donetsk in 2014, Igor Kozlovsky’s wife and younger son fled while he stayed behind to care for their older, disabled son, who could not be moved without special equipment. He settled into an isolated life, rising at dawn to practice yoga, write poetry and care for his son, even as the fighting intensified.

In January 2016, a gang grabbed him off the street. Later, in custody, he called his nephew to say he had been held in a basement and tortured. A military tribunal later found this confirmed pacifist guilty of “illegal possession of weapons” and threw him into one of Donetsk’s notorious prisons — hotbeds for tuberculosis. His health deteriorating, Kozlovsky was held for about a year in a damp, dark cell, then transferred to another prison in Horlivka.

Relatives have desperately tried to secure the 63-year-old’s release by engaging the few lawyers accredited to work in the DNR. Invariably, they have been cheated. Middlemen have offered to arrange his freedom for $100,000, but Denis Kozlovsky said he believes they are charlatans.

“If he was a guerrilla, then his arrest would be easier to accept. But he was man of peace, ­dedicated to finding common ground between religions,” he said. “Prisons in Donetsk are bad places, but my uncle is strong. Spiritually and psychologically, he is more prepared than most to survive.”

But the odds are stacked against prisoners such as Koz­lovsky; the process of swapping POWs is haphazard and poorly coordinated. The warring parties have failed to exchange data regarding detainees’ DNA, which would help establish the whereabouts of many missing people and relieve “the uncertainty and despair borne by their relatives,” the United Nations said.

Beyond these black sites, front-line communities face the threat of pillage. Troops have looted and commandeered homes of displaced residents, so some civilians endure the risk of shelling to protect their properties.

Troops on both sides of the line impose such harsh restrictions that those remaining in front-line areas are “isolated and fully dependent” on soldiers for water, food and fuel, the investigators say.

Brutal and vile treatment of prisoners, indefinite detention and the use of solitary confinement are common to both sides. But abuses occur on a greater scale in breakaway territory, where even the young are vulnerable. (Last August, five teenagers were detained by DNR security agents and have been held since.)

Investigators accuse Ukrainian law enforcement officers of systematically using torture and ill treatment to extract confessions from suspected separatists. These methods include extreme beatings, waterboarding, suffocation and electric shocks. Detainees are typically forced to sign a “testimony” that they have neither read nor written.

As one torture victim later explained, “Thinking about my finger being cut off was too much for me, so I told them what they wanted to hear.”

Investigations into complaints of torture are often corrupt and ineffectual, fueling a culture of impunity as perpetrators walk free.

“You need to be a kamikaze if you register your injuries,” one detainee, who had been tortured, told investigators. “If they learn about it, they will make you disabled and will deal with your family.”

For now, the fate of Aseev, the undercover journalist, is unknown. His last story — titled “Between Heaven and Hell” — was published from the eastern war zone on the same day as his disappearance. The young reporter knew the dangers but did the job regardless.

“Many will say it’s an unjustified risk. But, above all, I do this for myself,” Aseev said in an earlier interview with the Ukrainian newsweekly and website Zerkalo Nedeli. “This is my education, which cannot be purchased with any amount of ­money. . . . The old, timeworn notion that in war there are no winners is becoming a kind of mantra here.”