Russian media also cited unconfirmed claims that overall casualties could be as high as 200, the majority Russians.
The attack comes just days after USA forces for several hours used drones and B-52 bombers against several hundred "pro-regime" fighters equipped with artillery and a tank in Deir el-Zour province, an area in eastern Syria where the last IS fighters have converged among oil fields.
Russian citizen and "Another Russia" activist Kiril Ananyev was killed in Syria as result of an airstrike launched by the US-led coalition in the countryside of Deir Ezzor last week, the group's coordinator Alexander Averin told RIA Novosti news agency.
Top Photo | Russian soldiers walk outside a hospital in Deir el-Zor, Syria, Sept. 15, 2017.
One of the dead was named as Vladimir Loginov, a Cossack from Russia's Kaliningrad exclave.
The US-led coalition was set up in 2014 to battle Islamic State fighters in both Syria and Iraq, which were largely defeated previous year.
A representative of the Syrian Foreign Ministry, Ayman Soussan, said during Tuesday's press conference "we are not bound by anything that is formed by foreign sides, whatever their name or state, we are not bound by it, and it is of no concern to us".
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.
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". The Russians later called their us counterparts to ask if the strikes had ended so that the attacking force could recover the dead and wounded, the officials said, speaking on condition of anonymity.
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Analysts have grown increasingly concerned that the conflict in Syria could spill well beyond its borders, given the number of powers engaged in the country.
"I demand an explanation as to why Russian nationals take part in ground military operations in Syria, despite the statements by the president and defense minister that Russian military formations will be withdrawn from this country", Yavlinsky said. "All the media reports, though, need to be verified".
He said the situation in Syria was not discussed during a phone conversation between Putin and US President Donald Trump on Monday. Moscow's defense ministry issued a statement denying the involvement of any Russian service members.
Dan Shapiro, a former United States ambassador to Israel and a fellow at Tel Aviv University's Institute for National Security Studies wrote in an email on Tuesday that a clash could happen, but wasn't inevitable.
"Of course it's bad when Russians die from American weapons and we can't say it will have a good impact on our relations", said Vitaly Naumkin, a senior adviser to the Russian government on Syria.
A Syrian military officer says about 100 Syrian soldiers were killed on February 7 and 8.
The US military said it destroyed the tank after it moved within firing range of the US-backed forces while being supported by artillery fire.
Defence Secretary Jim Mattis said Russia has told the USA there were no Russians in the area of the strike.
US Defense Secretary General James Mattis has said that the US was puzzled by the attack near Deir Ezzor last week. "It doesn't make sense, and I think that's probably why the Russians, too, at least appear to be perplexed by it".