Russian soldiers who control an enormous nuclear power plant in Ukraine are detaining its workers and subjecting them to brutal interrogation and kidnappings as they search of saboteurs and partisans, forcing many to flee and raising concerns over plant safety, Ukrainian nuclear officials have told western media.

Both the Wall Street Journal and The New York Times have carried reports on a range of abuses Ukrainian officials say are being visited on the staff of the Zaporizhzhya nuclear plant —the largest of its kind in Europe — which was overrun by Russian forces on March 4.

The plant and its six Soviet-built reactors lies in southern Ukraine in the city of Enerhodar on the eastern side of the Dnipro River. This puts it across from territory still held by Ukrainian forces, according to the Times report. With 11,000 workers, the plant occupies a strategically important spot, and safety concerns at the plant make any Ukrainian counteroffensive to retake the area complicated.

The Times says Russian forces have fortified the outside of the plant with trenches and heavy artillery. Inside the plant, troops are stepping up measures to find anyone they think might pose a threat, according to local and plant officials who spoke with both newspapers.

“People are being abducted en masse,” Enerhodar’s exiled mayor, Dmytro Orlov, said during a meeting on Wednesday with officials from Energoatom, the state nuclear company oversees the complex. “The whereabouts of some of them are unknown. The rest are in very difficult conditions: They are being tortured and physically and morally abused.”

The Journal documented similar incidents in interviews with unnamed plant workers, their relative and other witnesses. On many days, they told the paper, several employees are handcuffed and driven off for questioning. Many come back visibly injured after days of interrogation, these people said. More than a dozen have disappeared altogether, they told the paper.

Some 500 Russian soldiers patrol the plant, these sources told the Journal, reprimanding workers who speak in Ukrainian rather than Russian and screening their cellphones for evidence of allegiance to Kyiv.

The International Atomic Energy Agency, the UN’s nuclear watchdog, has implored both the Russian and Ukrainian sides to be allowed to send a safety mission to the plant, but both have turned it down.

The Zaporizhzhya site houses some 30,000 kilograms of enriched uranium and 40,000 kilograms of plutonium as byproducts of burning nuclear fuel, and the Agency has said a visit to the plant would assure those inventories are intact.

Moscow’s plans for the plant remain unclear. Alexey Likhachev, head of Rosatom, Russia’s state nuclear corporation, told state media that his company does not plan to take operational control of the complex — despite the reported presence of Rosatom officials at the site.

But Russian Deputy Prime Minister Marat Khusnullin on a visit to Russian-occupied Ukraine said that Kyiv may soon be forced to pay Moscow for the power produced at the plant — which amounts to a fifth of all electricity production in the country

Exiled mayor Orlov said in an interview with a local radio station this week that many plant employees and other residents were trying to escape to Ukrainian-held territory. “Even young people are leaving the city,” he said, according to the Times. “It is unclear who will operate the nuclear power plant.”

Petro Kotin, the acting president of Energoatom, was quoted by the Times as saying that the plant’s “seizure and gradual transformation into a military base with many weapons and explosives” amounted to an act of “nuclear terrorism.” He said the company would continue to support its employees in occupied territories in every way it could.

Last month, Kotin raised concerns about the Russian militarization of the facility.

“Zaporizhzhya nuclear power plant is a well-fortified facility even in peacetime,” he said, according to the paper. “It is a perfect military base. Moreover, the Russians understand that the amount of nuclear material that is located there protects them. Ukraine will not strike at such an object.”