Over five days, 76 members of the regime and their Syrian and non-Syrian allies have been killed, including nine Russian soldiers, according to the rights group.
The deadliest was on Wednesday, when IS focused a bunch of Syrian and allied Russian fighters close to the city of Mayadeen within the japanese province of Deir Ezzor.
A Russian soldier standing guard in a central street in Deir Ezzor is one of a rumoured 4,000 or 5,000 Russian military servicement stationed in Syria.
On May 23, the same day the attack reportedly took place, Russian Defense Ministry spokesman General Sergey Rudskoy held a briefing in Moscow on the situation in Syria.
Among the 35, the Observatory claimed nine Russians were killed.
The remaining 26 have been all Syrian forces, he advised AFP. This would suggest that the other five Russians were mercenaries, which have been popularly used by Syria, particularly in the Deir Ezzor Province.
The Russian ministry said that the dead were military advisers attached to a Syrian army unit in the Deir el-Zour.
The ministry also said that 43 militants were killed in the same fighting.
Totals of armed workers have also passed away ever since Moscow activated its interference in backing of President Bashar al-Assad back in 2015.
Two military advisers died on the spot, while two others succumbed to their wounds in a hospital, the statement added. Russian ground and air forces have been crucial in helping Assad recapture most territory lost in the early stages of the nation's seven-year-long civil war.
Terrorists attacked Syrian and Russian troops, and the latter was forced to respond, according to the MoD.
Syrian and Russian forces participated in the fight, which took place at night, the ministry said, without giving a specific date.