Two of the group, whose identity was kept anonymous, said fighters were paid the equivalent of £3,000 a month before being dropped into Syria by Russian military planes to fight rebel factions – including ISIS.
Private military companies are banned under Russian constitution, but the two men insist they were trained by the company at a former Russian Special Forces base in Molkino that was handed over to them.
Russian private military company Wagner is hiring Russians to fight in Syria
They both claim that they were recruited after responding to vague job advertisements posted online.
One of the men, Alexander (not his real name) told Sky News: “Most people who go there for the money end up dead. Those who fight for ideals, to fight against the Americans, American special forces, some ideology – they have a better chance of survival.
“There were 564 soldiers with me and we were put up at the base. We had two reconnaissance companies, one air defence company, two assault groups and foot troops plus heavy artillery, thanks and so on.”
Dmitry added: “Approximately 500 to 600 people have died there.
“No one will ever find out about them… That’s the scariest thing. No one will ever know.
“We arrived at night at the airport. Then we were put in trucks. To be honest I was scared. I don’t have a strong build and I wasn’t very good at the drills.”
The official number of Russians killed in Syria stands at 19. However, the two fighters revealed that they believe hundreds of their fellow employees have been killed.
With private military companies banned in Russia, Alexander accused Wagner of covering up the death of its troops.
He continued: “Who will ever tell you about this? Sometimes the bodies are cremated but the papers say ‘they’re missing’. Sometimes the documents say the soldier was killed in Donbass. Sometimes they say ‘car accident’ and so on.”
Wagner is believed to have been grouped by a former special forces soldier, known as Nikolai Utkin.
The only known picture of Mr Uktin was published in a St Petersburg-based newspaper, Fontanka earlier this year.