Southwest of the city of Bakhmut, Ukrainian soldiers Andriy and Borisych live in a candle-lit bunker cut into the frozen earth.
For several weeks they have been confronting hundreds of fighters belonging to the Russian private military contractor Wagner throwing themselves against Ukrainian defences.
Disguised in a balaclava, Andriy recounts one seemingly endless firefight when they came under attack by a flood of Wagner fighters.
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"We were fighting for about 10 hours in a row. And it wasn't like just waves, it was uninterrupted. So it was just like they didn't stop coming."
Their AK-47 rifles became so hot from constant firing, Andriy says, that they had to keep changing them.
"It was about 20 soldiers on our side. And let's say 200 from their side," he says.
The Wagner way of war is to send a first wave of attackers that mainly comprises raw recruits straight out of Russian prisons.
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They know little of military tactics and are poorly equipped. Most just hope that if they survive their six-month contract they can go home rather than back to a cell
"They make the group - let's say from 10 soldiers - reach 30 metres then they start digging in to keep the position," Andriy says of Wagner.
Another group follows, he says, to claim another 30 metres.
"That's how, step by step, (Wagner) is trying to move forward, while they lose a lot of people in the meantime."
Only when the first wave is exhausted or cut down do Wagner send in more experienced combatants, often from the flanks, in an effort to overrun Ukrainian positions.
Andriy says facing the assault was a frightening and surreal experience.
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"Our machine gunner was almost getting crazy, because he was shooting at them. And he said, I know I shot him, but he doesn't fall. And then after some time, when he maybe bleeds out, so he just falls down."
Andriy compares the battle to a scene out of a zombie movie.
"They're climbing above the corpse of their friends, stepping on them," he says.
"It looks like it's very, very likely that they are getting some drugs before attack," he says, a claim that CNN has not been able independently to verify.
Even after the first waves were eliminated, the attack continued as the Ukrainian defenders say they ran out of bullets and found themselves surrounded.
"The problem was that they went around us. And that's how they surrounded us. They came from the other side. We didn't expect them to come from there.
"We were shooting until the last bullet, so we threw all the grenades we had and left only me and a few guys. We were helpless in that situation."
They were lucky. Held off until the last moment, the Ukrainian fighters say, Wagner withdrew at the end of the day.
Andriy's account of Wagner's approach matches that of a Ukrainian intelligence report obtained by CNN last week.
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According to that report, if Wagner forces succeed in taking a position, artillery support allows them to dig foxholes and consolidate their gains. According to Ukrainian intercepts, coordination between Wagner and the Russian military is often lacking.
CNN reached out to Wagner Group boss Yevgeny Prigozhin this week about allegations of abuse in the company's ranks.
Prigozhin responded in a statement that was largely sarcastic in tone via his press service, calling CNN an "open enemy" before insisting Wagner is an "exemplary military organization that complies with all the necessary laws and rules of modern wars."
As he speaks to CNN, the fields above Andriy's bunker reverberate to almost constant shelling. The whine of outgoing artillery is followed by a distant thud a few seconds later and a few kilometers away.
The chatter of small arms fire erupts as Ukrainian soldiers detect what they believe to be a Russian drone and try to bring it down.
Andriy's unit says it captured one Wagner fighter, whose story is as tragic as Wagner's tactics are primitive and brutal.
According to a recording of the man being questioned, the man is an engineer but had taken to selling drugs to make some money. He volunteered to join Wagner in the belief it would expunge his criminal record so that his daughter would have fewer problems following her dream to become a lawyer.
"And when did you realise, you are just meat?" Andriy asks him.
"At the first combat mission. They brought us to the frontline on December 28. They sent us forward last night."
"How many people were in the group?"
"Ten," he answers.
Andriy says he had told the engineer: "Obviously, you know that you will be killed (in battle). But you're afraid to fight for your freedom in your country."
"He said, 'Yes, this is true. We're afraid of Putin.'"
Andriy contrasted Russian President Vladimir Putin with Ukrainian President Volodymyr Zelensky, who not so long ago was the country's leading comedian.
"Our advantage is that yes, we do, we really can choose the guy whom the [Russians] call a clown. But as we can see, now, this guy is really the leader of the free world, at the moment, on our planet.
Andriy, who is from the southwestern city of Odesa and joined up within days of Russia's invasion, says that no matter how many more fighters are sent to storm their positions, they will resist.
"Most of my guys, they are volunteers. They had (a) good business, they had (a) good job, they had a good salary, but they came to fight for their homeland. And it makes a great difference," he says.
"This is the war for freedom. It's not even the war between Ukraine and Russia. This is a war between a regime and democracy."
