Four Russian military personnel were killed in fighting in eastern Syria's Deir al-Zor province, Interfax news agency reported on Sunday, citing the Russian Defence Ministry.
The fighting erupted after several groups of rebels attacked an artillery battery of the Syrian army, the ministry said.
Two Russian military advisers who directed the fire of the Syrian battery were killed on the spot. Five other personnel were injured and taken to a Russian military hospital. Two of them died from their wounds.
The ministry said that 43 rebels had been killed in the same fighting, Interfax reported.