The U.S. -backed Kurdish-led forces have been vying with Russian-backed Syrian troops reinforced by Iranian-supported militias for control of the oil-rich Deir el-Zour province.
The US-led coalition said it repelled that attack near the Euphrates River by hundreds of troops aligned with Syrian President Bashar al-Assad, who were backed by artillery, tanks, multiple-launch rocket systems and mortars.
Alexander Averin, a member of the Other Russia nationalist party, confirmed that Kirill Ananiev, a party member who left for Syria about a year ago, had been killed in the airstrike, noting that there were other "substantial losses".
"If there has been mass deaths of Russian citizens in Syria, then the relevant authorities, including the general staff of the Russian armed forces, have a duty to inform the country about this and decide who bears responsibility", Yavlinsky, who is running against Putin in next month's election, said on Twitter. Some 2,000 U.S. forces remain on the ground in Syria, allied to the Kurdish-led SDF alliance, which holds the largest swath of territory still outside the control of the government.
Vitaly Naumkin, perhaps Russia's most prominent Syria expert, also urged restraint in a news conference at the Valdai Discussion Club.
"Now we hear completely different explanations from our U.S. partners regarding their presence in Syria - they say that this presence should be maintained not only until military tasks are accomplished, but also until there is a stable political process that should end in a transition of power which will be acceptable for all - meaning for the United States - which is regime change".
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But the Kremlin along with Russian President Vladimir Putin refuse to admit anything about the deaths. Russia's military said it had nothing to do with the attack and the US accepted the claim.
What are Russia's casualties in the Syrian conflict?
"We only handle the data that concerns Russian forces servicemen", Dmitry Peskov, the Kremlin spokesman, said at a news briefing Tuesday. They counted on the US having too little time to target them without risking casualties among its mainly Kurdish allies, but strikes began when only half the force had made it to the base, Kommersant newspaper reported, citing a former comrade of the mercenaries.
Mr Yavlinsky said fierce mercenary forces "are trained at Russian army facilities" and "receive military awards in the Kremlin" in secret.
Major Adrian Rankine-Galloway, a Pentagon spokesman, said the coalition had spoken with Russian Federation via a special hotline prior to the strike "to avoid misperceptions and miscalculations that could endanger each other's forces". I can't give you anything on that. Putin and Trump spoke by phone on Monday, but reportedly did not discuss the Syria incident.
Some of those Russians killed were said to be mercenaries for the secretive Russian private military group Wagner, which has provided thousands of contract soldiers to help the Assad regime. He noted that the Russian military could have been unaware of the contractors' involvement in the attacking force because coordination between them has been very poor.