Four Russian military personnel have been killed in fighting in eastern Syria's Deir al-Zor province, Interfax news agency reports.
The fighting erupted after several groups of rebels attacked an artillery battery of the Syrian army, the Russian Defence 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 43 rebels had been killed in the same fighting, Interfax reported.
Syrian and Russian forces participated in the fight, which took place at night, the ministry said, without giving a specific date.
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Russia is the main military backer of Syrian President Bashar al-Assad in his country's multi-sided civil war.
The intervention has tilted the war in favour of al-Assad's forces, who have in recent months made territorial gains against rebels and militants including Islamic State.
