Both the US-trained 'minister of war' Gulmurod Khalimov, who has a $3 million bounty on his head, and the high-profile ISIS commander, Abu-Muhammad al-Shimali were among 'four influential field commanders are confirmed to be dead', the Russian defence ministry said in a statement posted on Facebook.
"As a result of a precision air strike of the Russian air forces in the vicinity of Deir Ezzor city, a command post, communication centre and some 40 ISIS fighters have been killed. According to confirmed data, among the killed fighters are four influential field commanders including Deir Ezzor emir Abu Mohammed al-Shimali," it said.
Abu-Muhammad al-Shimali also known as Tarad Muhammad al-Jarba was reportedly killed in a Russian air strike in Syria. Al-Shimali (also known as Tarad Muhammad al-Jarba) is believed to have helped six of the 2015 Paris suicide bombers to travel to Syria and back for training.
The Iraqi-born Saudi Arabia citizen, who defected from Al-Qaeda to IS in 2015, had a $5 million bounty on his head following the attacks.
Khalimov, who is known as the IS group's minister of war and the highest-ranking defector from ex-Soviet Tajikistan, suffered been 'fatally wounded', the ministry added.
Reports of Khalimov's death have surfaced before and the Tajik interior ministry said it could not immediately confirm the claim.
"We are working with our Russian colleagues to obtain reliable information," a spokesman told AFP.
But a spokesman for the Tajik security services, speaking to AFP, suggested that 'this time around' he might have been killed.
"We're checking the information," he said.
The news of both men's deaths was met with skepticism.
Twitter user @KyleWOrton posted: "That man is Tarad al-Jarba and he was promoted some time ago. Let's wait and see if Russia actually got him."
In 2016, the United States (US) offered a $3 million bounty for information leading to Khalimov's location or arrest. Russia's SU warplanes dropped 'bunker buster' bombs on the fighters as they were meeting near Deir Ezzor to discuss how to respond to the advance of the Syrian army, Moscow said.
Backed by Russia, Syrian troops on Tuesday broke through a years-long siege imposed by IS militants on tens of thousands of civilians in Deir Ezzor.
The Times reported in April that Khalimov, described as the highest-ranking IS commander in Mosul, had been killed in an airstrike. The trained sniper and former colonel was apparently wounded in 2015 but survived.
He headed the Tajik interior ministry's special forces unit and received American training before joining IS in 2015, pledging allegiance to the jihadist group in a video released in May 2015.
In the footage he warned that he and other ISIS recruits based in the Middle East were 'coming' for top officials in the mainly Muslim Tajikistan, including long-ruling President Emomali Rahmon.
Last year, his second wife, herself a former interior ministry official, fled Tajikistan with her three young children to join Khalimov in Syria.
His eldest son, 18-year-old Bekhruz, also tried to join his father in Syria but was detained at the Dushanbe airport, and sentenced this summer to 10 years in prison.
In July, police in Tajikistan killed four of Khalimov's relatives in a gun battle, an interior ministry source has said, and three other relatives were detained.
The source claimed that all of those killed or detained were IS 'supporters' and said they were intending to flee to neighbouring Afghanistan, but did not offer any proof to back up the claims.