Russia has admitted four of its soldiers were killed in the clash with what it called "terrorists." Jihadists had targeted Syrian and allied Russian fighters near the town of Mayadeen in eastern Syrian.
"Several mobile terrorist groups attacked Syrian government artillery at night," on Wednesday, killing at least four Russian and nine Syrian soldiers, the Russian Defense Ministry announced on Sunday.
"Two Russian military advisers, who controlled fire of the Syrian battery, died at the scene," the ministry said. Two more soldiers died later at a Russian military hospital. The eastern town of Mayadeen, just south of Deir el-Zour, had been an Islamic State stronghold until it was recaptured by Syrian forces last October.
The ministry claimed that 43 rebels were killed. The militants appear to have represented a variety of Islamist and other groups but Russia refers to them all as "terrorists."
The number of rebel casualties could not immediately be confirmed, but an independent monitor claimed the casualty rate among Russian and Syrian fighters appeared to be higher than the Kremlin has reported.
The UK-based Syrian Observatory for Human Rights said at least nine Russian and 26 Syrian troops were killed during the attack. The Observatory has been a fairly reliable source of war casualties throughout Syria's seven year civil war.
"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 on Sunday. The Observatory has reported a number of attacks on Syrian forces in Deir al-Zour in recent weeks.
Syrian President Bashar al-Assad gets significant military support from Russia in the form of troops and punishing airstrikes, but there are growing reports of private Russian mercenaries also engaged in fighting on the ground.
If confirmed, the death of nine Russian fighters would represent about 10 percent of all Russian casualties since Moscow entered the conflict in 2015.
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bik/jm (AFP, AP, Reuters, dpa)