A Russian military doctor said around 100 had been killed, and a source who knows several of the fighters said the death toll was in excess of 80 men.
The timing of the casualties coincided with a battle on Feb. 7 near the Syrian city of Deir al-Zor where, according to U.S. officials and associates of the fighters involved, U.S.-led coalition forces attacked forces aligned with Moscow’s ally, Syrian President Bashar al-Assad.
Russian officials said five citizens may have been killed but they had no relation to Russia’s armed forces.
The clashes show Moscow is more deeply involved in Syria militarily than it has said, and risks being drawn into direct confrontation with the United States in Syria.
The casualties are the highest that Russia has suffered in a single battle since fierce clashes in Ukraine in 2014 claimed more than 100 fighters’ lives. Moscow denies sending soldiers and volunteers to Ukraine and has never confirmed that figure.
The wounded, who have been medically evacuated from Syria in the past few days, have been sent to four Russian military hospitals, according to five sources familiar with the matter.
The military doctor, who works in a Moscow military hospital and was directly involved in the treatment of wounded men evacuated from Syria, said that as of Saturday evening there were more than 50 such patients in his hospital, of which around 30 percent were seriously wounded.
The doctor, who spoke on condition of anonymity because he is not allowed to disclose information about casualties, said at least three planeloads of injured fighters were flown to Moscow between last Friday and Monday morning.
He said they were flown back on specially equipped military cargo planes which can each accommodate two or three intensive care cases and several dozen less severely wounded patients.
Maria Zakharova, a spokeswoman for the Russian foreign ministry, said initial information was that five Russian citizens in the area of the battle may have been killed, but they were not Russian troops. She said reports of tens or hundreds of Russian casualties were disinformation inspired by Russia’s opponents.