Most of those killed and injured were Russian and Ukrainian, many of them veterans of the separatist conflict in eastern Ukraine, according to Alexander Ionov, who runs a Kremlin-funded group that fosters ties to separatists and who's personally fought alongside pro-government forces in Syria.
A Kremlin spokesman said on Wednesday he could not rule out that there were Russian civilians in Syria, but that they had no connection to the Russian armed forces.
The reports of Russian deaths in the incident in Deir al-Zor come amid persistent tension in U.S.
A Cossack organization in Kaliningrad posted on social media that one of its members had also been killed, and the independent Russian newspaper Novaya Gazeta, citing unnamed sources, reported that 13 Russians were killed and another 15 wounded.
At the time of the February 7 attack, the Russian defence ministry said "there are no Russian servicemen in Deir-Ezzor".
Lieutenant General Jeffrey Harrigian, who heads up Air Force operations for the region, told Pentagon reporters: "We detected and saw a tank that took a shot at us".
Vitaly Naumkin, perhaps Russia's most prominent Syria expert, also urged restraint in a news conference at the Valdai Discussion Club.
Of course, if indeed Russian soldiers were killed while fighting under covert circumstances - in the same way as killed U.S.
Putin has emphasized that no uniformed Russian soldiers were killed in the strike. Russia's military said it had nothing to do with the attack and the USA accepted the claim.
'No survivors': Russian plane crashes with 71 people on board near Moscow
A Tu-154 miitary transport crashed into the Black Sea in 2016, killing all 92 people aboard in a disaster blamed on pilot error. The domestic plane was being operated by Saratov airlines, and had 65 passengers on board, along with six crew members.
The Syrian conflict, which began as an uprising against Bashar al Assad's government during the Arab Spring in 2011, has ravaged the country and turned into a 16-year war.
CIA Director Mike Pompeo also refused to confirm specifics when asked whether the USA was responsible for killing Russian contract soldiers in Syria during a hearing on Tuesday, but said, "We have seen in multiple instances foreign forces using mercenaries in battles that will begin to approach the United States".
The strike was the second by the US against pro-Syrian regime forces last week, according to the USA military's Central Command. Utkin was photographed next to President Vladimir Putin at a Kremlin reception in late 2016, held to honor him and others for their service to Russian Federation, for which they have received state awards, according to Peskov.
Mr Yavlinsky said fierce mercenary forces "are trained at Russian army facilities" and "receive military awards in the Kremlin" in secret.
The Russian citizens are fighting in Syria for a private military company, according to numerous reports. Russian media reports suggest that Wagner, a private Russian military contractor, sent the soldiers to guard energy assets on behalf of Assad.
"The call for an immediate cessation of hostilities ... has gone unanswered", al-Za'tari said. The Pentagon said no US or SDF forces were killed by the tank.
Wagner, which is under United States sanctions for supporting separatists in eastern Ukraine, has several hundred contractors in Syria, according to analysts.
Moscow launched a military operation in Syria in support of President Assad on 30 September 2015.
Private contractors - why are they in Syria?
The investigative team said the total number of Russian victims of the airstrike is unlikely to be established, but said 'it is beyond doubt that this incident indeed took place'.