One fighter was shot twice, sent from the hospital back to the front, where he drank melted snow to live. Forced to assault Ukrainian positions repeatedly, until a grenade blinded him. Saved from the trenches by a doctor who made him a hospital orderly.

Another was jailed at 20 for minor drugs charges, sent to the front aged 23. Given almost no training, he was dead three weeks later - among likely 60 Russians killed in an assault on the very day Russian President Vladimir Putin celebrated the defeat of the Nazis in Red Square.

These two stories, of remarkable survival and premature death, epitomize the squalid and exhausting loss of life in Russia's trenches. Yet there is one distinction: the dead are prisoners, promised respite from their jail terms if they join so-called Storm-Z battalions run by the Russian Ministry of Defense.

Life expectancy is short, conditions themselves tough to survive, and convicts describe being used as cannon fodder. Tens of thousands of convicts have been recruited to serve at the front line, at first by the mercenary group Wagner -- a scheme then taken over by the defense ministry.

CNN spoke to the mother of one convict, Andrei, who was jailed aged 20 on drug charges and sent to the front line as part of the Russian military's recruitment program. The mother provided extensive video, documentation and chat messages to verify her son's story, and his early death, just three weeks after deployment.

CNN also spoke to a rare survivor of the Storm-Z units, Sergei -- who was first interviewed by phone in a military hospital months earlier and last week recounted the savage and deteriorating life in the Russian trenches.

While the appalling fighting conditions are well known, much Russian testimony is from prisoners of war, and provided through Ukrainian facilitators. These two stories represent rare testimony delivered directly from Russians. CNN has changed the names and removed key details from these two accounts for the safety of the interviewees.

Sergei now works two jobs to keep his family fed, but said he is still waiting for military compensation for his multiple injuries. His ears ring at night from the shell shock, making it hard to sleep in the silence of his home.

He said he was concussed nine times from artillery shells landing nearby while on the front line, over an eight-month period. Last winter he was shot in the leg, then sent back to the front after 10 days' treatment, he said. He was shot again, in the shoulder, and properly hospitalized. Two months later, a manpower shortage meant he was sent again to the front lines, where he said he found convict amputees had been given radio duties, and troops were discarding their bulletproof vests as they had minimal protective value.

"They don't help against shells, since their [Ukrainian] artillery strikes with high accuracy," Sergei said. "Our artillery can fire three or four times, and God willing something explodes. It's crooked and most often, it hit us first."

Quotidian horrors

The casualty rates are hard to conceive. Sergei said that from his unit of 600 prisoners recruited in October, only 170 were still alive and all but two of them wounded. "Everyone was injured, two, three, some four times," he said. He recalled seeing colleagues blown apart by shells landing close to them, and his marvel at surviving. One assault was particularly vivid.

"I remember most clearly the last of the nine concussions I had," he said. "We attacked. RPGs, drones few at us. Our commander yells on the radio, 'I don't care, go ahead! Don't come back until you take this position!' Two of us found a small foxhole and dived in there."

But their ordeal was not over. "A (Ukrainian) drone threw a grenade at us, and it landed in the 30-centimetre gap between us. My friend was covered with shrapnel all over. Yet I was untouched somehow. But I lost my sight for five hours -- just a white veil in front of my eyes. They carried me out by hand."

He finally found doctors who took pity on him, giving him a job as a hospital orderly -- moving corpses, checking bodies for identifying papers, cleaning -- until the last month of his contract was served out.

Sergei recalls the quotidian horrors of the Russian trenches. Food was mostly tinned meat with instant noodles added, but water was the hardest to obtain. "You have to walk three to four kilometers to get it. Sometimes we didn't eat for several days, we didn't drink for several days." He said in winter they would survive by drinking melted snow. "It wasn't very pleasant, but we had to."

Discipline was maintained through executions, he said. "Sometimes the commander 'reset' people. He zeroed them out, killed them. I only saw it once -- a fight with a man who stole and killed his own people in the trenches. I didn't see who of the four people around him shot. But when he tried to escape, a bullet hit him in the back of the head. I saw the head wound. They carried him away."

'Just about freedom'

For Andrei, the horrors at the frontline were short-lived. His mother Yulia described how he was "not yet a man" when he was sent, aged 23, to the front line. His voice messages -- joking about the weather -- and boyish looks in uniform, betray a young heart caught in an ugly world.

She said: "He didn't remember the amount of money he was offered, said he hadn't checked. So, I didn't see any financial interest for him. It was just about freedom. He had a long term, nine-and-a-half years, and he had served three."

Yulia shared a video of Andrei on a training ground in occupied Ukraine, briefly learning assault tactics. His poorly shaven face was pictured in still images, sunburned, under a large camouflage helmet, in the back of an army truck. The images were few, as his time on the front was short.

It was on May 8 that Andrei messaged his mother to say his unit was being sent to the front, one of the most feverishly contested parts of the eastern battlefield. The assault would begin at dawn, on May 9 - a feted day in modern Russian history when the Kremlin marks the anniversary of the defeat of the Nazis by the Soviets with the pomp and grandeur of a military parade across Red Square. Putin presided over a cut-down version of the ceremony this year, which analysts attributed to so much of Moscow's arsenal being damaged or deployed to the Ukrainian front.

Yulia tearfully recalled that last exchange. "We were arguing. It is horrible to say, but I already thought of him like he was dead. He left (Russia) knowing everything. Every day I told him 'no, no, no.' And he didn't listen to me. When he said, 'we're going to storm,' I wrote him 'Run, Forrest, Run.'"

Then, like so many convicts with limited cellphone access on the front lines, he vanished completely. In the weeks since, Yulia learned from the relatives of the other prisoners recruited from his penal colony that up to 60 had died in that one assault -- a number hard to corroborate, but in keeping with the extraordinary casualties reported by observers from these units made up of convicts.

Yulia received no body, or belongings, only a letter from the Ministry of Defense which records Andrei's death as being the day he left prison for the front lines.

"The hardest part was that I was afraid, he would kill someone," Yulia sobbed. "Ridiculous as it sounds, I was afraid he would go through all this and come back to me as a murderer. Because I can live with my son as a drug addict, but with my son as a murderer -- it was difficult for me to accept it."

At times, the horrors Russia's invasion inflicts on Ukraine are almost matched by what it does to its own.