However, he added that some isolated pockets of resistance could remain in the area.
Speaking at an annual briefing for foreign defense attaches in Moscow, Gerasimov said: "All IS units in Syria have been destroyed, and the territory has been liberated".
State news agency SANA said late Wednesday that Syrian troops and allied forces had captured around two dozen towns and villages on the west side of the river.
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 Britain-based Syrian Observatory for Human Rights said the strikes were to support the Kurdish-led Syrian Democratic Forces (SDF).
Buhari Visits Kano, Grants Amnesty to 500 Prison Inmates
Buhari urged the ex-convicts to be of sound character, and live a healthy life devoid of crime and criminality. The inmates including men and women were selected from the Kurmawa Central Prison, Kano.
Russian Federation supports Syrian President Bashar al-Assad's government and provides air support for government forces.
Russian President Vladimir Putin has said that the Islamic State (IS) extremist group has suffered a "complete defeat" along the Euphrates River in eastern Syria.
Earlier on December 6, a monitoring group said that Russian warplanes overnight hit a village controlled by IS extremists in Deir al-Zor Province, killing at least 21 civilians, including children.
"However, as a result of the coalition's air strikes, Raqqa was 90% destroyed and became actually unsuitable for living", the chief of Russia's General Staff said.
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