Allegations of brutal interrogations, where Ukrainian men were shot and threatened with rape, have been made by a former Russian military officer.
Konstantin Yefremov, the most senior officer to speak openly, told the BBC in an exclusive interview Russia now sees him as a traitor and defector.
At one site in southern Ukraine, he said "the interrogations, the torture, continued for about a week".
"Every day, at night, sometimes twice a day."
Mr Yefremov tried to resign from the army numerous times - but he ended up being dismissed for refusing to return to Ukraine. He has now fled Russia.
KONSTANTIN YEFREMOV
Image caption,
One of the last photos of Konstantin Yefremov in his army uniform, Chechnya, June 2022
Mr Yefremov shows us letters from the military.
In the first document, he is accused of "shirking his duties" and disregarding an order to return to Ukraine. It is described as "a serious breach of discipline".
The second letter refers to Mr Yefremov's "early dismissal from military service… for breaking his contract".
"After 10 years of service I was denounced as a traitor, a defector, just because I didn't want to kill people," he says. "But I was glad that I was now a free person, that I wouldn't have to kill or be killed."
One of Konstantin Yefremov's military documents explaining his dismissal from the Russian army - it says he "committed a gross disciplinary offence"
Mr Yefremov was out of the army. But not out of danger of being sent back to the war.
In September 2022, President Putin declared what he called "partial mobilisation". Hundreds of thousands of Russian citizens would be drafted into the military and sent to Ukraine.
Mr Yefremov says he knew - because he had already served with the military in Ukraine - he would not be left alone. He came up with an escape plan.
"In the house where I was living I made a hatch in the attic ceiling… in case police and enlistment officers broke in to deliver call-up papers.
"Enlistment officers were driving to my house and waiting for me in their cars. So, I rented a flat and hid there.
"I hid from the neighbours, too, because I'd heard of cases when neighbours told police about young men who'd been drafted and were hiding. I found this situation humiliating and unacceptable."
Mr Yefremov contacted Russian human rights group Gulagu.net, which helped him leave Russia.
What does Mr Yefremov think about those Russians - and there are many - who express support for Vladimir Putin's decision to invade Ukraine?
"I don't know what's going on in their heads," he says. "How could they allow themselves to be fooled? When they go to market, they know they could be short-changed. They don't trust their wives, their husbands.
"But the man who has been deceiving them for 20 years, he only has to give the word and these people are ready to go and kill and die. I can't understand it."
As we end our chat, Mr Yefremov says sorry to the people of Ukraine.
"I apologise to the entire Ukrainian nation for coming to their home as an uninvited guest with a weapon in my hands.
"Thank God I didn't hurt anyone. I didn't kill anyone. Thank God I wasn't killed.
"I don't even have the moral right to ask for forgiveness from the Ukrainians. I can't forgive myself, so I can't expect them to forgive me
