Islamic State has suffered a "complete defeat" in eastern Syria at the hands of Syrian troops and Kurdish-led forces, both supported by Moscow, Russian President Vladimir Putin said.
Mr Putin said Russian defence minister Sergei Shoigu told him that operations against IS on both the western and eastern banks of the Euphrates River had been successfully completed.
The president said some isolated pockets of resistance could remain in the area.
Russian general Valery Gerasimov told foreign military attaches that "all IS gangs on the territory of Syria have been destroyed and its territory has been freed".
Gen Gerasimov said in a statement carried by Russian news agencies that Syrian government forces coming from two directions met in eastern Syria on Wednesday, completing the route.
He said "there is no area under IS control in Syria", but the group is believed to still maintain a presence in some scattered areas.
The Russian military said it has provided air support to Kurdish forces and local tribes in the oil-rich province of Deir el-Zour in eastern Syria and helped co-ordinate their offensive against IS.
The Kurdish-led Syrian Democratic Forces on Sunday thanked both the US and Russia for their military support, days after the US announced it would stop arming the group.
Russia launched an air campaign in support of Syrian president Bashar Assad’s forces in 2015.
Syrian activists said air strikes on Tuesday killed at least 12 civilians in an eastern Syrian village held by IS.
Deir Ezzor 24 said the attack targeted the village of al-Jarthi.
The Britain-based Syrian Observatory for Human Rights said 21 civilians were killed, among them nine children.
It said Russia carried out the strikes.