The Russian foreign ministry stressed on Tuesday that the combatants were not Russian servicemen and that no Russian military equipment was involved in the fighting.
The ministry had formerly admitted five Russian nationals were killed.
Russia's assistance has proved to be a major success as Syria has managed to liberate key areas over the past years, including the second largest city of Aleppo, relying on Russia's air cover and military advice.
Yet although the clashes earlier this month around Deir Ezzor killed perhaps dozens of Russians, the aftermath has been marked not by escalation but by Moscow's reticence to highlight an awkward fact of its Syrian war: growing reliance on a mercenary army.
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Lavrov pointed to the military and political debacle in Iraq since the United States invasion as an example of what can happen when the USA meddles in the region.
The Foreign Ministry said "several dozen" Russians were injured and an unspecified number died in USA airstrikes during the clashes on the night of February 7.
But details of the operation have gradually emerged, along with growing anger inside Russian Federation that its citizens' deaths are being quietly ignored as inconvenient bad news ahead of President Vladimir Putin's run for re-election next month. Afghan war will cost B in 2018 USA expands air campaign to northern Afghanistan Mattis defends plans for new nuclear capabilities MORE said "don't ask" him who directed the attack, but cast doubt on the idea that the mercenaries were working on their own.
Three sources familiar with the matter told Reuters last week that about 300 men working for a Kremlin-linked Russian private military firm had been either killed or injured in a clash in Syria with the US-led coalition.
"But I doubt that 257 people all just decided on their individual own selves to suddenly cross the [Euphrates] River into enemy territory and start shelling a location and maneuvering tanks against it", he added. Was it local directions? "Don't ask me. I don't know", he said, according to a Pentagon transcript. "It hasn't in this case, thankfully, but it's unsafe".