Moscow's defence ministry said Sunday four of its servicemen were killed in clashes in Deir Ezzor
Russian fighters were among dozens of pro-government forces killed in eastern Syria this week in a deadly wave of attacks by Islamic State group jihadists, Moscow and a monitor said on Sunday.
After the collapse of its so-called "caliphate" last year, IS now only holds tiny pockets of Syria, mainly in the vast desert stretching to its eastern border.
This week, the jihadists ramped up their hit-and-run attacks on regime positions there, according to the Syrian Observatory for Human Rights war monitor.
The deadliest was on Wednesday, when IS targeted a group of Syrian and allied Russian fighters near the town of Mayadeen in the eastern province of Deir Ezzor.
"There were 35 pro-government forces killed, including at least nine Russians. Some of those Russian nationals were government troops, but not all of them," said Observatory head Rami Abdel Rahman.
The remaining 26 were all Syrian forces, he told AFP.
A steadfast ally of President Bashar al-Assad, Moscow has helped his army recapture swathes of territory since 2015 by providing air strikes and ground troops.
There are also widespread reports of private Russian mercenaries on the ground.
Moscow's defence ministry said Sunday four of its servicemen were killed in clashes in Deir Ezzor.
Two were military advisers supporting Syrian artillery operations and died immediately, and another two died of their wounds in a Russian-operated military hospital in Syria.
Three others were wounded.
Russia did not specify when, where, or whether IS was involved, but it appeared to be the same incident as the IS attack reported near Mayadeen.
The jihadist group itself claimed it attacked regime forces in eastern Syria on Wednesday.
The assault was the largest in series of IS guerilla raids on regime positions this week.
On Tuesday, 26 regime forces were killed in a surprise IS attack in desert areas of the neighbouring province of Homs, according to the Observatory.
And a pair of IS assaults between Saturday night and Sunday morning killed at least 11 pro-regime forces in Deir Ezzor.
"The latest attack brings to 76 the number of Syrian troops and allied Iranian and Russian forces killed since the escalation," Abdel Rahman said Sunday.
He said the uptick came the day after the last IS fighters were bussed out of southern parts of Syria's capital Damascus, including the ravaged Palestinian camp of Yarmuk, in a negotiated withdrawal.
Many headed towards the Badiya, the stretch of Syrian desert extending from Homs province through Deir Ezzor to the eastern border with Iraq.
The Observatory said the evacuated fighters were actively involved in the recent attacks.
"IS is trying to take the initiative and show it can still threaten the regime and its allies despite the losses it suffered in other areas," said Abdel Rahman.
Government positions in the Badiya make for an easy target: they are few and far between, so reinforcements take a long time to arrive.