Ministry spokeswoman Maria Zakharova says those killed in Syria are not Russian service personnel.
Five Russian citizens were likely killed in a US-led coalition bombing on pro-regime Syrian fighters last week, Moscow said on Thursday, as it acknowledged non-military casualties in the attack for the first time.
"Based on preliminary information, as a result of an armed encounter, the reasons for which are now being investigated, we could be talking about the deaths of five people," the Russian Foreign Ministry spokeswoman, Maria Zakharova told a briefing.
"The assumption is that these are Russian citizens. But all that needs to be checked, in particular, of course, their citizenship, are they indeed citizens of Russia, or of other countries," she said.
"These are not Russian service personnel," Zakharova said, referring to the casualties in February 7 clashes near the Syrian city of Deir Ezzor.
Associates of Russian private military contractors fighting alongside regime forces in Syria have said there were large-scale casualties among the contractors when US-led coalition forces clashed with pro-regime forces in Deir Ezzor province on February 7.
Zakharova said media reports about dozens or hundreds of Russian dead in the Syria clashes were "disinformation". She added that US actions in Syria were probably designed to undermine the country's territorial integrity.