Before he was killed by a sniper in Syria at age 23, Ivan Slyshkin wrote a poignant message on social media to his fiancee: "We will see each other soon — and I will hold you as tight as I possibly can."

Key points: About 3,000 Russians under contract to the Wagner group fought in Syria since 2015

About 3,000 Russians under contract to the Wagner group fought in Syria since 2015 The secret deployment has helped keep the official death toll low

The secret deployment has helped keep the official death toll low About 73 private contractors have been killed in Syria, according to Fontanka

But Mr Slyshkin's name won't be found among the Russian Defence Ministry's official casualties in the fight against Islamic State extremists.

That's because the young man who left his hometown of Ozyorsk in the Ural mountains was one of thousands of Russians deployed to Syria by a shadowy, private military contractor known as Wagner, which the Government does not talk about.

Mr Slyshkin's gravestone depicts him holding a machine gun, according to a local news website Znak.com that sent a reporter to his March 2 funeral in Ozyorsk, where friends said he joined Wagner to earn money to pay for his wedding.

"He was in Wagner's group," his friend Andrei Zotov said, adding that Mr Slyshkin was killed as the security forces were advancing on the Al-Shayer oil field north of Palmyra.

"There are many good guys there. He volunteered to join the company.

"Like many Russian fighters, he wanted to solve his money issues."

The St Petersburg-based website Fontanka reported that about 3,000 Russians under contract to the Wagner group have fought in Syria since 2015, months before Russia's two-year military campaign helped to turn the tide of the civil war in favour of Syrian President Bashar Assad, a longtime Moscow ally.

When Vladimir Putin went to a Russian air base in Syria on Monday and told Russian troops that "you are coming back home with victory", he did not mention the private contractors.

Russian troops are expected to remain in Syria for years while the contractors are likely to stay to guard lucrative oil and gas fields under a contract between the Syrian Government and another Russian company allegedly linked to a businessman known as "Putin's chef" for his close ties to the Kremlin.

Proxy fighters such as Mr Slyshkin have played a key role in Syria.

Russian death toll lower than reality

Mr Putin addressed troops at a Russian air base in Syria on Monday. ( AP: Mikhail Klimentyev, Sputnik, Kremlin Pool Photo )

In addition to augmenting troops officially sent by Moscow, their secret deployment has helped keep the official Russian death toll low as Mr Putin seeks re-election next year.

The Russian Defence Ministry has said 41 of its troops have died in Syria.

But according to Fontanka, another 73 private contractors have been killed there.

The Kremlin and the Defence Ministry have stonewalled questions about Russians fighting in Syria in a private capacity.

Private contractors have been used by countries such as the United States in Iraq and Afghanistan for years; Russian law forbids the hiring of mercenaries or working as one.

But Russia has used such proxies before — in the conflict to help pro-Moscow separatists in eastern Ukraine since 2014.

One Russian commander boasted of working alongside Russian troops who said they were "on vacation" while fighting in Ukraine.

'When people die, it doesn't have to be announced'

Russia supported the Syrian Government in its fight against Islamic State. ( AP )

The Defence Ministry has refused to say how many of its troops are in Syria, although one estimate based on absentee ballots cast in the Russian parliamentary election last year indicated 4,300 personnel were deployed there.

That number probably rose this year because Moscow sent Russian military police to patrol "de-escalation zones".

"The Russian people are not very enthused by the idea of an empire that would involve their boys coming home in body bags," said Mark Galeotti, senior researcher at the Institute of International Relations in Prague.

"There's clearly a lack enthusiasm for this conflict.

"By having this military company Wagner, they can have a force they can actually deploy … but when people die, it doesn't have to be announced," Mr Galeotti said.

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Fontanka, which is respected for its independent reporting, has obtained what it described as Wagner's spreadsheets and recruitment forms that indicate thousands of Russians have fought in Syria.

Of about 3,000 Wagner employees deployed to Syria over the years, the single largest contingent at a given time has been about 1,500, Fontanka reporter Denis Korotkov said.

Since 2015, at least 73 of them have died, he said.

Another investigative group, Conflict Intelligence Team, or CIT, put the number of private contractor deaths at 101. Both outlets say those are conservative estimates.

"The most important proof is people, dead and alive, who have said they are mercenaries and their relatives say there are mercenaries," CIT founder Ruslan Leviev said.

"How would hundreds of people all over the country collude and come up with the same story?"

Activists with CIT made a name for the group by combing social media and other records for Russia's involvement in Ukraine and are doing the same for Syria.

Survivors receive generous compensation to keep silent

Moscow sent Russian military police to patrol "de-escalation zones" this year. ( AP: Nataliya Vasilyeva )

Both Fontanka and CIT published photos from what they called a Wagner training base in the Krasnodar region of southern Russia.

Some of the facilities look identical to those seen in official Defence Ministry photos of a military base in Molkino, in the same area.

Agreements signed with the security companies have kept the private contractors and members of their families from speaking to the media about their activities.

Survivors receive generous compensation for keeping silent, and most attempts by AP to contact relatives and friends of those killed have been unsuccessful.

Media in southern Russia reported the death of Alexander Karchenkov in Syria in November 2016.

The BelPressa website showed the mayor of his hometown of Stary Oskol giving a medal to Mr Karchenkov's tearful widow, Marina, and mother.

Marina Karchenkova said her husband went to Syria as a contract soldier because he "had children to raise".

In a handwritten form dated December 2015, Mr Karchenkov said he was unemployed, serving in the Soviet army in 1989-1991 and as a volunteer in Kosovo in 1998-2000.

In October, the Islamic State group released video of two Russian captives it said had been fighting in Syria, and one of them identified himself as Roman Zabolotny and said the other was Grigory Tsurkanu.

The Defence Ministry denied they were Russian servicemen, and media reports said they were working for Wagner.

Their fate at the hands of the extremists is unknown.

Wagner group founder under US sanctions

St Petersburg entrepreneur Yevgeny Prigozhin (left) has been dubbed "Putin's chef". ( AP: Alexei Druzhinin, Sputnik, Kremlin Pool Photo )

The Wagner group was founded by retired Lieutenant Colonel Dmitry Utkin, who came under US sanctions in June after the Treasury Department said the company had recruited former soldiers to join the separatists fighting in Ukraine.

He was photographed a year ago at a Kremlin banquet thrown by Mr Putin to honour military veterans.

Also under US sanctions is Yevgeny Prigozhin, the St Petersburg entrepreneur dubbed "Putin's chef" by Russian media because of his restaurants and catering businesses that once hosted the Kremlin leader's dinners with foreign dignitaries.

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In the more than 10 years since establishing a relationship with Putin, his business expanded to other services for the military.

Earlier this year, an anti-corruption foundation run by opposition leader Alexei Navalny detailed how Mr Prigozhin's firms have come to dominate Defence Ministry contracts.

The US State Department put Mr Prigozhin on its sanctions list in 2016 related to the Ukrainian conflict, citing his "extensive business dealings" with the Defence Ministry.

Among the firms linked to Mr Prigozhin is Evro Polis, a Moscow-registered company that Fontanka reported has become a front for Wagner's operations in Syria.

In 2016, Evro Polis listed the sale of food products as its core activities, according to the Spark Interfax database.

But this year, it listed mining, oil and gas production, and opened an office in the Syrian capital of Damascus.

Wagner's commercial motive

AP obtained a copy of a 48-page contract between Evro Polis and Syria's state-owned General Petroleum Corp, which said the Russian company would receive 25 per cent of the proceeds from oil and gas production at fields its contractors capture and secure from Islamic State militants.

While the five-year contract could not be authenticated, Fontanka reported the same deal in June.

"The link between Evro Polis and Prigozhin is significant and is not in doubt," Fontanka's Korotkov said.

"We believe that this firm is just a cover for the private company Wagner, and it could be an attempt to legalise this group, possibly for a commercial use later on."

Both Evro Polis and Mr Prigozhin's Concorde Management and Consulting were unavailable for comment, and the Defence Ministry did not reply to AP's request for comment.

An AP reporter who went to Evro Polis' Damascus office in November found it closed, with no sign on the door.

Syria's Ministry of Petroleum and Mineral Resources declined comment when asked about the Evro Polis deal.

Asked about the contract, the Russian Energy Ministry told Fontanka it cannot divulge "commercial secrets", and declined comment to the AP.

As the Russian campaign in Syria draws to a close, the private contractors will probably stay, analysts say.

Wagner "is likely to cement its footing because we saw that there were not only military goals to pursue … but there is a commercial motive," Mr Leviev said.

"Someone needs to guard the oil fields."

Thousands of Russians were deployed to Syria by a shadowy, private military contractor known as Wagner. ( AP: Nataliya Vasilyeva, file photo )

AP