Russia's foreign ministry has confirmed that "several dozen" Russian citizens - not regular soldiers - were killed or wounded in a recent battle in Syria.
The statement did not give a date. Previously the Russian government said "probably five" Russians had died in a clash in Deir al-Zour province.
Some reports have said more than 100 Russian mercenaries were killed in US air strikes on 7 February.
Russia does not divulge firm details of "volunteer" fighters in the conflict.
What does Russia say?
The latest Russian statement said no Russian regular forces had been involved in a "recent armed clash".
The foreign ministry spoke of "several dozen" citizens from Russia and other ex-Soviet republics, who have been killed or wounded. It said the injured are undergoing medical treatment at a number of hospitals.
"It has been pointed out that certain Russian citizens in Syria have arrived there of their own free will and for different reasons. The foreign ministry does not have the authority to assess the validity and legality of their decisions," it added.
Who are the mercenaries?
Officially, Russia's military role mainly takes the form of air strikes, helping President Bashar al-Assad's forces.
However, independent media have found evidence of other Russian citizens fighting for the Syrian government side.
Relatives of the hired fighters say they have been paid insurance money by unknown representatives of so-called private military companies, which bring their bodies back.
A pensioner from the north-west Arkhangelsk Oblast region, Nina Atyusheva, told the BBC she has been fighting to get the state to formally record her son's death: "I want to ask this question: in what way are the children sent by the armed forces different from those like mine, who went as a volunteer?"
Read more: The Russian guns for hire in Syria
What happened on 7 February?
The US launched air and artillery strikes in Deir al-Zour province, which, it says, killed 100 militia.
It says it had to defend the headquarters of its allies, the Syrian Democratic Forces (SDF), which was under attack by fighters aligned with the Syrian government.
The US says these forces crossed an informal demarcation line along the Euphrates River; the government controls the western side and the SDF the east.
The two sides have clashed over the past year while trying to drive Islamic State (IS) militants from their last major stronghold in the country.
The Pentagon said it had been in constant contact with Russia in the run up to the February operation.
The Syrian government called the attacks a "massacre" and said it would complain to the United Nations.
Russia and the US have accused each other of operating in the area and planning to control nearby oil fields.
How many Russians have died in Syria?
According to the Russian defence ministry, 44 official military personnel have died in Syria.
The number of dead private military contractors remains unknown.
A Reuters investigation showed that at least 131 Russians died in Syria in the first nine months of 2017 - a number thought to include private military contractors.