MOSCOW -- The parents of one of two purported Russian ground fighters captured in Syria by Islamic State (IS) militants have told RFE/RL they were instructed by the Federal Security Service (FSB) "not to make a fuss" about their captivity.

The claim threatens to draw attention to a potentially awkward development for the Kremlin, which has publicly limited Russia's military campaign there to air strikes but has been dogged by reports that it is secretly waging war on the ground through mercenaries working for a company called Vagner.

Working as a mercenary is illegal under Russian law.

But the brother of one of the apparent captives told RFE/RL that his brother had been fighting for Vagner, saying, "They send them to slaughter -- lure them with money and then abandon them -- because professional soldiers can't fight like they do."

On October 3, the IS propaganda arm Amaq published a video purporting to show two Russian soldiers captured in Syria. Both men appeared in grubby gray overalls with bruising on their face, one with a heavily swollen eye.

In the clip, one man identifies himself as Roman Zabolotny and says he was born in 1979 in the southern Russian region of Rostov. He identifies the man sitting silently next to him as Grigory Tsurkanu, born in 1978 in the Domodedovo region of Moscow Oblast.

The men are said in the video to have been captured near Deir al-Zor, where the Syrian Army with Russian air support this week launched a major offensive on IS strongholds.

The Russian Defense Ministry has denied the men in the video are Russian soldiers, while the Foreign Ministry has said it is trying to establish their identities.

President Vladimir Putin's spokesman Dmitry Peskov said on October 4 that "of course Moscow is concerned about the fate of these citizens if they are Russian citizens."

The next day, Peskov declined to comment, telling Interfax, "Naturally the possible circumstances of their capture [and] the confirmation of their identities -- this is something for the corresponding agencies to handle."

'Sent To Slaughter Without Respect'

In comments to RFE/RL's Russian Service on October 4, Roman Tsurkanu said the man in the video is his brother, Grigory. He said his brother had traveled to the Middle East in May and was fighting for the private military contractor Vagner.

Tsurkanu said he believed his brother had joined Vagner to earn money to support his two children. He described a highly secretive arrangement in which fighters are required to keep their employment secret.

"If there even is a contract, then no one sees it, no one knows where it is kept. They sign a nondisclosure document. The only thing I know is that they describe everything beforehand: that [in the case of death] medals and money will be returned to relatives."

Roman Tsurkanu complained that his brother had superior combat skills but was used as cannon fodder. "I just don't understand why we keep contractors there, why we can't make our own elite troops from them,...why they use them like meat, and why it's those who have combat qualities, who are real fighters. What's more, it's illegal, they're sent to slaughter without respect."

Mercenaries are illegal in Russia, and rumors of the existence of Vagner have repeatedly been denied. In August, an image was posted on social media showing Putin meeting Dmitry Utkin, who has been identified in multiple reports as the founder of an unregistered private military-contracting agency called Vagner. Putin spokesman Peskov confirmed the authenticity of the photograph, which was reportedly taken at a Kremlin reception in December.

'Don't Make A Fuss'

Roman Tsurkanu said his brother's departure in May was not his first tour in Syria and that he had also fought in eastern Ukraine in 2014 on the side of Russia-backed separatists against Ukrainian forces.

Moscow eventually acknowledged its covert invasion of Crimea in 2014 but has repeatedly denied sending troops and weapons to the ongoing conflict zones in Ukraine, despite evidence of official involvement. A shaky Ukrainian cease-fire was signed in February 2015.

"They were sending them packing, only Comrade Putin, dammit, signed a cease-fire," Roman Tsurkanu said to suggest that the Russians were handily defeating the pro-Kyiv forces.

Reached by telephone, Roman Tsurkanu initially agreed to speak to RFE/RL's Russian Service in person and on camera at his family home but then stopped taking calls.

RFE/RL's Russian Service spoke to Roman and Grigory's parents, both pensioners, who said they had been instructed by phone "not to make a fuss" about incident. They later said the call had come from the FSB.

"We will wait," the Tsurkanus told RFE/RL, "If they're not freed, we'll call you, journalists."

Cossack From Rostov

A group of bloggers known as the Conflict Intelligence Team identified the other man in the video, Zabolotny, as a Cossack from the southern Rostov region. The Don Cossacks confirmed in comments to Interfax that Zabolotny was a Cossack from the Rostov region, saying: "The Cossacks are praying that Roman comes home. The publication of this video with him is a shock."

The blogger group also suggested that both Zabolotny and Tsurkanu were mercenaries. Comparing open-source photographs, the group also said Zabolotny had likely taken part in an attack in April on a campaign office in Rostov set up by opposition politician Aleksei Navalny.

Anatoly Kotlyarov, a deputy in the Rostov-on-Don City Duma, told Interfax that he had recently met Zabolotny, although he said he was not closely acquainted with him. On October 5, Kotlyarov said he had "received information from a reliable source that he has been executed in Syria. I would like it not to be so, but at the moment that is the information."

Written by Tom Balmforth in Moscow based on reporting by RFE/RL's Russian Service