Some 120 ISIL fighters and 60 foreign mercenaries were killed in a series of Russian air strikes in Syria over the past 24 hours, the defence ministry in Moscow said on Saturday.
Puzzlingly, the ministry also said three senior ISIL commanders, including Omar Al Shishani, had been confirmed dead as a result of an earlier Russian strike.
Moscow reported Al Shishani's death despite the fact that the Pentagon said in 2016 the notorious fighter had been killed by American troops in Iraq.
"A command post of the terrorists and up to 80 [ISIL] fighters including nine natives of the Northern Caucasus were destroyed in the area of Mayadeen" in Syria's east, the ministry said, adding that another 40 ISIL fighters were killed around the nearby town of Albu Kamal.
Regime forces backed by Russian air raids on Friday broke into Mayadeen, one of ISIL's last bastions in Syria.
In another air strike, more than 60 foreign mercenaries from the former Soviet Union, Tunisia, and Egypt were killed in the Euphrates Valley south of Deir Ezzor.
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The ministry said "large numbers of foreign mercenaries" were coming into the Syrian border town of Albu Kamal from Iraq.
It also said Russian forces had killed senior ISIL commanders Omar Al Shishani, Alaa Al Din Al Shishani and Salah Al Din Al Shishani, all natives of the Northern Caucasus.
Moscow reported their deaths after taking "several days" to confirm the results of an earlier strike on the northern outskirts of Albu Kamal which destroyed an ISIL command post with more than 30 fighters, including the natives of the Northern Caucasus.
'Omar the Chechen'
The Pentagon announced in March 2016 that American forces had killed Al Shishani, one of the most notorious faces of ISIL known for his thick red beard.
Four months later, his death was confirmed by the extremist group itself.
Al Shishani, whose nom de guerre means "Omar the Chechen", came from the former Soviet state of Georgia's Pankisi Gorge region, which is populated mainly by ethnic Chechens.
He fought as a Chechen rebel against Russian forces before joining the Georgian military in 2006, and fought Russian forces again in Georgia in 2008.
He later resurfaced in northern Syria as the commander of a group of foreign fighters and became a senior leader within ISIL.
The Russian defence ministry was not immediately available for further comment.
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Rami Abdel Rahman, head of the Britain-based Syrian Observatory for Human Rights, disagreed with Moscow's report.
"Salah Al Din Al Shishani is still alive and is somewhere in the regions controlled by jihadist groups in the west of Aleppo province. He is a famous commander, and his jihadist group is allied with the jihadists of the Al Nusra Front but only in their fight against the regime," he said.
"He has no links with ISIL."
The advances against ISIL in Deir Ezzor have resulted in a heavy civilian death toll through Russian and coalition air raids.
The Observatory said Russian air strikes on Thursday night killed 14 people, including three children, fleeing across the Euphrates on rafts near Mayadeen.
Since it intervened in Syria in 2015, Russia has not acknowledged any civilian deaths from its strikes, and it dismisses the Observatory's reporting as biased.
Moscow has been staging air strikes in support of its ally Damascus, targeting both ISIL in Deir Ezzor province and rival jihadists led by Al Qaeda's former Syria affiliate in Idlib province in the north-west.
Turkey announced on Saturday it had launched a "serious" operation in Idlib province with Ankara-backed Syrian opposition forces.
The operation had been highly expected in the province after last month's talks in the Kazakh capital of Astana during which Turkey, Iran and Russia agreed on setting up "a de-escalation zone" in Idlib.
In late September, Russian president Vladimir Putin visited Turkey where he discussed the situation in Syria with Turkish president Recep Tayyip Erdogan; earlier this week, Mr Erdogan went to Tehran.
The Turkish president said the operation was a "new step" to establish security in Idlib, promising Ankara would not desert civilians there.
"Today, there is a serious operation in Idlib and it will continue," he said at his political party's conference in western Turkey's Afyonkarahisar province.
"The Free Syrian Army (FSA) is leading an operation in Idlib right now," he added, referring to moderate rebel groups in Syria. He said the Turkish military was not yet in the province, however.
Turkey-backed Syrian forces are fighting the extremist alliance Hayat Tahrir Al Sham, which is led by former Al Qaeda affiliate Jabhat Fatah Al Sham. In recent weeks, the Turkish military has been dispatching tanks and armoured vehicles to the border with Idlib.
A Syrian rebel commander speaking from Turkey said no military operations are ongoing at the moment but that preparations were underway for Turkish troops and FSA fighters to enter Idlib.
"The aim of the operation is to implement the Astana agreement by setting up Turkish observation posts similar to those of Russia," Lieutenant Colonel Fares Al Bayoush said.
"This cannot be achieved without confronting the Nusra Front," he said, referring to Jabhat Fatah Al Sham by its former name. "The aim is to finish Nusra Front."
Mr Erdogan said Turkey would provide security inside Idlib and Russia on the periphery.