Last week the USA unleashed an air and artillery barrage on Syrian government-backed troops after some 300 attackers launched what appeared to be a coordinated assault on Syrian opposition forces accompanied by USA advisers in the province of Deir el-Zour.
The Russian Defense Ministry, which insisted that its troops weren't involved in the incident, said 25 Syrian volunteers were wounded in the USA strike.
Many of the Russians killed or injured were veterans of the Ukraine conflict, according to Alexander Ionov, the head of a Kremlin-funded organization that fosters ties to separatists who's fought alongside pro-Assad forces in Syria.
Russian news media reported that they had confirmed the deaths of several other military contractors, as well.
"We are focused on a singular enemy - ISIS", said the Air Force general.
But he would not comment on whether Russian Federation has influence over the forces that did attack.
Vitaly Naumkin, perhaps Russia's most prominent Syria expert, also urged restraint in a news conference at the Valdai Discussion Club.
At no point, an American military spokesman said, was there any chance of direct conflict between United States and Russian forces.
Both sides are fighting the last remnants of the Islamic State group in Syria.
More details about those killed in the strikes began to emerge in the following days.
He said: "They are, in essence, special troops of the Russian army - despite the fact that mercenaries and private military companies are officially banned in Russia".
The Kremlin said on Tuesday it had no information about Russian mercenaries reportedly being killed in last week's attack.
The strike was the second by the US against pro-Syrian regime forces last week, according to the USA military's Central Command.
President Trump (L) with Russia's President Vladimir Putin.
The Kremlin and the Russian Defence Ministry have refused to comment on media reports on Russian private contractors in Syria.
What about the Russian mercenaries?.
Several Russians who had been hired as paramilitary contractors were among more than 100 men killed in United States airstrikes in northern Syria last week, according to their friends and families. After these calls, he said the coalition officials gave the approval to conduct "strikes to destroy hostile forces".
USA officials added that the Russian government had lodged no complaint about its citizens being killed. Citing unverified preliminary reports "from Syria" he claimed that as many as 644 could have been killed.
The rift deepened after Turkey launched an operation targeting the YPG in the Kurdish enclave of Afrin in northern Syria on January 20, despite calls by the US for restraint. "It doesn't make sense, and I think that's probably why the Russians, too, at least appear to be perplexed by it". In recent days, rumors swirled online in Russian Federation that large numbers of those military contractors had perished under American fire.