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

But Slyshkin’s name won’t be found among the Russian Defense 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 doesn’t talk about.

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 told The Associated Press, adding that 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,” Zotov said. “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 favor of Syrian President Bashar Assad, a longtime Moscow ally.

When 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 like Slyshkin have played a key role in Syria. In addition to augmenting troops officially sent by Moscow, their secret deployment has helped keep the official Russian death toll low as Putin seeks re-election next year.

The Russian Defense 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 Defense Ministry have stonewalled questions about Russians fighting in Syria in a private capacity. Private contractors have been used by countries like 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.