Russia's defence ministry on Thursday said its mission to oust Islamic State jihadists from Syria had been "accomplished" with the country "completely liberated" from the extremist group.

"The Russian armed forces' goal to defeat armed groups of the ISIL terrorist organisation in Syria has been accomplished," said senior military officer Sergei Rudskoi, using an alternative acronym for the group."There is not a single village or district in Syria under the control of ISIL. The territory of Syria has been completely liberated from fighters of this terrorist organisation," he told reporters.

There has been an "unprecedented" involvement by Russia's airforce in recent days, he said, with warplanes making 100 sorties and staging up to 250 strikes daily.

At the same time, special forces were active on the ground directing planes and "destroying the most odious leaders of militant groups behind enemy lines," he said.

The Syrian Observatory for Human Rights, which monitors the conflict, said Thursday that IS still holds about eight percent of Deir Ezzor province.

Rudskoi said "separate sabotage bands of ISIL" could still be operating but would be fought by Syrian government troops, indicating that Russia's involvement would be scaled down.

"With the liquidation of armed bands of the ISIL terrorist group in Syria, the Russian contingent will concentrate its main efforts on providing aid to the Syrian people in rebuilding peace" and ensuring ceasefire commitments were met, he said.

Russia began its bombing raids in September 2015 in support of Syrian President Bashar al-Assad's beleaguered forces.

Those strikes have helped Assad regain control over much of war-ravaged Syria.

Last month, President Vladimir Putin said efforts to end the war were entering a "new stage" as the focus shifts from military intervention to political reform.

More than 340,000 people have been killed since the conflict broke out in March 2011 with protests against Assad's rule that sparked a brutal crackdown.