Maria Zakharova, the Russian Foreign Ministry spokeswoman, emphasized in a briefing in the capital Moscow on Thursday that the five victims weren't Russian servicemen.
"According to preliminary information, five people, presumably Russian citizens, may have been killed in combat, the circumstances of which are being clarified," media outlets quoted Zakharova as saying.
"Some others have been injured, and it's still necessary to check whether they are citizens of Russia or other nations," the spokeswoman noted.
This marks the first time Russian and US combatants have become directly engaged in combat in Syria.
The US-led coalition in Syria attacked pro-Damascus forces in Dayr al-Zawr on February 7 in what it claimed to be defensive strikes.
The US military claimed in a statement that the operation came after some 500 pro-government forces attacked a group of US-backed opposition forces in the region. It said US advisers had a presence among the opposition group.
In the days after the strike, US Defense Secretary Jim Mattis and other US officials, however, said they had no information on Russian casualties. The U S officials also said the Russian government had lodged no complaint about its citizens being killed.
Russia has repeatedly accused the US of trying to hamper a Syrian full victory against Daesh, a Takfiri terrorist group that is on its last legs in eastern Syria and in neighboring Iraq. Moscow has released evidence showing that US forces stationed in Dayr al-Zawr’s border regions and in Jordan have been collaborating with Daesh and other militants through providing intelligence about Syrian and Russian forces.
Russia has backed Syria’s fight against terror since September 2015 when Russian air force and troops arrived in the Arab country at an official request of Damascus to help the government fight against Daesh and other Takfiri militants.
Russia’s assistance has proved to be a major success as Syria has managed to liberate key areas over the past years, including the second largest city of Aleppo.
The US and its allies back militants fighting to topple the Syrian government. They have also been bombarding what they call Daesh positions inside Syria since September 2014 without any authorization from the Damascus government or a UN mandate.
The US strikes, however, have on many occasions resulted in civilian casualties and failed to fulfill their declared aim of countering terrorism.
Syria’s ambassador to the UN said on Thursday the prime objective of the US-led military coalition is supporting terrorists operating in the Arab country.
Bashar Ja’afari said in a statement to the UN Security Council that the US presence in Syria violated the 15-member body's resolutions and the UN Charter.