"According to preliminary information, we could be talking about the deaths of five people - presumably Russian citizens - as a result of an armed confrontation whose causes are being examined", Maria Zakharova, a spokeswoman for the Russian Foreign Ministry, told reporters at a press briefing Thursday.
The US had said the coalition acted in self-defence when an enemy unit of 300 to 500 fighters launched an attack on an established Syrian Democratic Forces [SDF] position east of the Euphrates river in Deir az-Zour province.
A Russian military doctor said around 100 had been killed, and a source who knows several of the fighters said the death toll was in excess of 80 men.
Russian authorities deny deploying private military contractors to Syria.
The battle, wherein as many as 500 or so combatants loyal to the Syrian government were said to have advanced toward a known U.S. position in western Syria and fired with tanks and artillery, reportedly ended with up to 300 attackers killed by USA airpower and artillery.
Her claims were backed by Russian prosecutor-general Yuri Chaika, who said foreign agencies' attempts "to interfere in our country's domestic affairs require a harsh reaction from our prosecutors".
She said her son previously fought in two wars in Russia's Chechnya region but later struggled to find work in their village.
The Russian defense ministry did not respond to questions from Reuters about casualties in Syria. Kremlin spokesman Dmitry Peskov said at the time that Utkin was invited as a decorated military veteran.
Bloomberg first reported this week that Russian military contractors took place in what the United States called an "unprovoked attack" on a well-known headquarters of the Syrian Democratic Forces, a rebel cohort the USA has trained, equipped, and fought alongside for years.
United States special forces were accompanying the SDF forces that came under attack, officials in Washington said.
He said that on February 7 the force had advanced toward the settlement of Khusham, in Deir al-Zor province, into a zone designated as neutral under a deal between the Russian military and the US-led coalition.
"The situation where no facts have leaked the public sphere over nearly half-a-year that the investigation is in progress is simply impossible for the U.S.", Lavrov said.
The Russian official insisted those killed in the clash were not members of the Russian armed forces.
The aim was to test if the US-led coalition would react.
"First of all the bombers attacked, and then they cleaned up using Apaches (attack helicopters)", Yevgeny Shabayev, a Cossack paramilitary leader with ties to Russia's military contractors, told Reuters. The US warned Russian Federation of the attack, but it's unclear if Russia's military passed on notice to the troops on the ground.
Mercenaries not directly affiliated with the Russian military may be convenient for Moscow's business interests in Syria while assuring deniability of government involvement.
"The warning was 20 minutes beforehand, in that time it was not feasible to turn the column around", said the source.