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.
Among the 35, the Observatory claimed nine Russians were killed. "A few of these Russian nationals have been authorities troops, however not all of them", stated Observatory head Rami Abdel Rahman.
According to him, the remaining 26 of the dead were in the forces controlled by Syrian President Bashar al-Assad.
The Syrian government, backed by Russian and Iranian military, now controls most of central Syria, including Aleppo.
Earlier on Sunday, the Russian Ministry of Defence said four soldiers were killed by rebel shelling in Deir Ezzor province after several rebel groups attacked a Syrian army artillery battery.
According to the statement, the Syrian armed forces with the assistance of the Russian military advisers took the fight started after the attack.
Flash Floods Devastate Ellicott City in Maryland
But the Maryland city around 13 miles west of Baltimore was hit Sunday by flooding that was even worse than 2016. Some are already asking whether enough was done after the last flood to prevent a similar catastrophe.
Five Russian soldiers were also injured and were promptly taken to the Russian military hospital, it said. 5 other Russians were wounded and 2 expired later due to their injuries.
Russian Federation entered the Syria conflict in September 2015, offering support to Damascus which changed the course of the war, allowing government troops to retake more than half the country from rebels and IS militants. It said the jihadists had targeted a group of Syrian and allied Russian troops in Deir Ezzor province on Wednesday.
Russian Federation has lost about 90 personnel in the civil war.
A Syrian military source based in the east told AFP that Assad's troops had cleared large parts of territory from IS, which was now lashing out. Syrian observers said that this is the same incident as the attack ISIS near Madina.
The defence ministry in Moscow said its forces had killed 43 militants in the fighting.