Although Moscow has resisted calls to specify details of the mission or how many casualties followed the attack, they have admitted that five Russian citizens may have been killed.
Valery Shebayev, a member of a Cossack group, visited some of those injured in a Moscow hospital and declared that the mission resulted in “a massacre”.
It is believed that an estimated 500 troops, most of which were comprised of military contractors working for a paramilitary company named Wagner, and a Christian militia group moved towards an oil and gas field that was controlled by the Syrian Democratic Forces (SDF) on February 7.
The group crossed the River Euphrates, near the Syrian city of Deir Ezzor, as part of the operation.
The details of the mission are still unknown, but when forces loyal to the regime started shelling a base held by the SDF, US forces retaliated with devastating airstrike and artillery fire.
The US is also believed to have sent an AC-130 attack aircraft and helicopters in response to the fire.
Ruslan Leviev, an activist for the Conflict Intelligence Team, declared that it is estimated that 20 to 30 Russian citizens died in the horrific incident.
Russia has not confirmed the reports and emphasised that it has “said everything” it wanted to.
Spokesman for the Kremlin, Dmitri Peskov, remarked: “We have no new information about this and we said everything we wanted to say on this matter.”
If the highest estimates of the casualty numbers are true, the attack would mark the deadliest clash between the two superpowers since the Cold War.
The US revealed that while the incident was taking place it attempted to contact Russia through the use of deconfliction channels.
US Defence Secretary James Mattis added that Moscow was “notified when the firing began”.
He continued: “The Russians professed that they were not aware when we called them about that force that had crossed [the Euphrates].
“As it came closer they were notified when the firing began.”
Mr Leviev believes the operation was an “independent initiative” that was given the go-ahead because it was believed the anti-regime forces in the country would have been distracted by ongoing Turkish operations in the northwest.
He went on: “The pro-Assad forces thought the Kurds were distracted by what’s happening in Afrin, so [used] the opportunity and try to take control of the Coneco oil and gas factory.
“It looks like it was an independent initiative.”
Grigory Yavlinsky, the liberal Yabloko Party’s nominee to challenge Mr Putin for the Russia Presidency, called for the Russian government to “announce” the number of casualties to the nation.
He said: “If massive Russian casualties took place, then relevant officials…must announce this to the country and find out who is responsible.”