Russia has been a major backer of Syria's President Bashar Assad whose government troops have been advancing in the eastern province of Deir el-Zour against IS under the cover of Russian airstrikes.
Meanwhile, the Kurdish-led Syrian Democratic Forces are also marching against the Islamic State group, backed by the US-led coalition.
Russia's Defense Ministry said in a statement early on Tuesday that its airstrikes just outside Deir el-Zour, on the eastern bank of the Euphrates River, killed more than 304 IS fighters and left more than 200 wounded.