It is believed that at least 150 people have died fighting for the “ghost army” but relatives of those who have ben killed in the conflict are now complaining about being kept in the dark about the circumstances.
Between 1,500 and 2,000 mercenaries, about half the size of Russia‘s official military contingent in Syria, are thought to have been inside the middle eastern country at peak moments since Mr Putin announced military operations in September 2015.
Wagner units are understood to have played a role in key battles against Islamic State (ISIS) forces whilst working under the wing of Russia’s armed forces.
However the existence of Wagner is denied by the Russian defence ministry who keep its operations secret.
Ruslan Leviyev, a Moscow activist who has investigated the forces, told The Times: “The purpose of this group is to act as Russia’s main ground force alongside Syrian government troops, allowing the Russian army to minimise casualties.”
Mr Putin said this month Russia was reducing the number of troops in Syria after their operation with President Assad’s forces resulted in “the total rout of the terrorists”.
Mr Leviyev, who runs a group called Conflict Intelligence Team, said: “Wagner was used so Russia could say to the West, look at our successful campaign, look how few deaths we had compared to you in Iraq or Afghanistan.”
Officially, only a few dozen Russian soldiers have died in the conflict.
Mr Leviyev and others pieced together evidence of Wagner’s casualties by examining social media, talking to relatives of those killed and taking leaked documents from the group.
Families of Wagner mercenaries killed or missing in the conflict are usually persuaded to keep quiet and given compensation pay-offs of £38,000 (3 million roubles).
But some are now speaking out after getting little or contradictory information about their relative’s fate.
The parents of Grigory Tsurkanu, a 38-year-old fighter, told their story in an interview with Moscow’s Dozhd independent television channel.
A bruised Mr Tsurkanu and another Wagner fighter, Roman Zabolotny, appeared in an Isis video last October saying that they had been captured. They have not been heard of since.
Mr Tsurkanu’s parents said that officers stayed in their home for two weeks after the video was published.
They told the couple that they needed “protection” and kept them from reporters. They now believe this was a ruse to stop them creating a fuss.
The couple made a request to the defence ministry for help in locating their son but have not received a reply.
A Wagner representative told them that he had been killed but could not provide details.
Mrs Tsurkanu told Dozhd: “I was naive. I lived my whole life being naive, believing people, and now I don’t believe any more. Putin once said: ‘We don’t abandon Russians.’ I believed it. Now I understand that was a lie. Damn them, Wagner, all of them.”
In another case, Mr Leviyev was contacted by the sister of Tamerlan Kachmazov, 29, a Wagner fighter who went missing at the end of September.
She went to Wagner’s training camp at Molkino, in Russia’s southern Krasnodar region, after her brother had been missing for two months.
Military experts say that it is close to a Russian special forces training base.
She was told there that her brother had been killed in Syria and Isis fighters had burnt his body.
Officers could not tell her where or in which battle he had died.
She said: ”It seemed like they were doing everything just to get rid of us. I supposed that he disappeared without trace and when we started to annoy them it was easier for them to just tell us he was dead.”
The group offered Mr Kachmazov’s family compensation and an empty zinc coffin but they refused in disgust.
Wagner is named after the nom de guerre of its commander, Dmitry Utkin, a former Russian special forces soldier aged in his late 40s with a reported interest in Nazi Germany.
While fighting as a mercenary is illegal in Russia, Wagner operates with Kremlin approval.
Mr Putin did not mention the outfit when he held an awards ceremony last Thursday for 600 servicemen and women who served in Syria.
Much of what is known about Wagner has been uncovered by Denis Korotkov, a former policeman who is a reporter for the news website Fontanka, based in St Petersburg. His sources say that the mercenaries in Syria were given assault rifles made in North Korea and Soviet-era weapons such as RP-46 machine guns. Wagner units have also been equipped by Syrian forces with T-72 tanks and 122mm howitzers.
The group’s level of co-ordination with the Russian army in Syria has wavered. While some fighters reach the war zone on defence ministry passenger planes, others take commercial airlines to Damascus, the Syrian capital Mr Leviyev said those fighters who were delivered to Russia’s permanent Hmeimim air base were then transferred to the front line by helicopter.
Mr Korotkov, writing on the Fontanka website, says he thinks that Wagner is fighting “on the side of good” against extremists in Syria but its uncertain status is unacceptable. “Taking direct part in battles is generally considered the prerogative of a state,” he says. “But when war is being waged by a company with an unclear chain of command, with unclear responsibility and with unclear powers, then I am deeply convinced that this is indecent.”
Russia’s government has said that it will retain forces at Hmeimim and its naval base at Tartus, despite declaring victory over Isis. Mr Korotkov said it was too soon to say what further role Wagner would play. “That’s as hard as predicting what will happen to Syria.”