Around 120 ISIS militants and almost 60 foreign mercenaries were killed following a series of Russian air strikes in Syria over the past 24 hours, the defence ministry in Moscow said on Saturday.
The ministry also confirmed the death of three senior ISIS commanders including Omar al-Shishani as a result of an earlier Russian strike.
"A command post of the terrorists and up to 80 ISIS militant including nine natives of the Northern Caucasus were destroyed in the area of Mayadeen," the ministry said.
Mayadeen is one of the ISIS group's last bastions in Syria.
"Another 40 militants were killed around the town of Albu Kamal," it added.
In another air strike, more than 60 mercenaries from the former Soviet Union, Tunisia, and Egypt were killed in the Euphrates Valley south of Deir Ezzor, according to AFP.
Moscow reported the deaths of the ISIS commanders after taking "several days" to confirm the results of an earlier strike on the northern outskirts of Albu Kamal.
However, Rami Abdel Rahman, head of the Britain-based Syrian Observatory for Human Rights, said "Salah al-Din al-Shishani is still alive and is somewhere in the regions controlled by militants groups in the west of Aleppo province. He is a famous commander, and his militants' group is allied with the militants of the Al-Nusra front but only in their fight against the regime," he said.
"He has no links with ISIS."
The Observatory said Russian air strikes on Thursday night killed 14 people, including three children, fleeing across the Euphrates on rafts near Mayadeen, AFP reported.