At first, Russian officials said only four Russian citizens - but perhaps dozens more - had been killed; a Syrian officer said around 100 Syrian soldiers had died.
The Syrian Observatory for Human Rights monitoring group said Islamic State fighters launched two new attacks in Deir el-Zour on Saturday near the town of Mayadeen and the town of Boukamal near the Iraqi border.
Among the 35, the Observatory claimed nine Russians were killed. "Some of those Russian nationals were government troops, but not all of them", said Observatory head Rami Abdel Rahman.
The remaining 26 were all Syrian forces, he told AFP.
Russian Federation has provided crucial military support to President Bashar Assad's forces, helping them to roll back mainstream rebels as well as the Islamic State group.
The statement added that two Russian military advisers were killed on the spot. 5 other Russians were wounded and 2 expired later due to their injuries. In result of this war, 15 lac people have permanent disabilities and eight six thousand people have lost their limbs.
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According to the ministry, the fight in the Syrian province of Deir-ez-Zor happened in the dark.
"Russia's armed forces and the Syrian army have defeated the most battle-hardened group of worldwide terrorists", Putin said.
He was part of a group of military instructors who were training Syrian soldiers.
According to the defence ministry statement, cited by AFP news agency, 92 Russian soldiers have been killed in Syria since Moscow entered the country's ongoing civil war in September 2015.
In an attack by Islamic state militants on Syrian army position near the town of Mayadin in the province of Deir ez-Zor were killed at least 26 soldiers of the government army, and nine Russians.