REUTERS

About 300 men working for a Kremlin-linked Russian private military company Wagner were either killed or injured in Syria last week, according to three sources familiar with the matter.

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, according to Reuters.

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.

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Read alsoVideo released of U.S. massive airstrike on Russian mercenariesThe 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.