Russian and U.S. media reports say scores of Russian mercenaries were killed when US forces carried out air strikes against a pro-regime force in eastern Syria last week.

According to Russian newspaper Kommersant, the contingent of a Russian private military company (or a private military contracting agency) Wagner Group that numbered about 600 mercenaries has been hired by Syrian president’s allies.

Scores of troops, mostly a mix of Russians and Ukrainians fighting on the side of Syrian President Bashar al-Assad, were reportedly killed while storming the base held by US and Kurdish forces in the Deir Ezzor region.

Bloomberg News quoted Russian sources as saying as many as 200 Russian nationals could have been killed in a clash last Wednesday and Thursday around the oil fields of Deir Ezzor region between forces loyal to Bashar al-Assad and a Kurdish-led opposition force supported by US advisers.

The New York Times said that Russian and Syrian officials had estimated that dozens of Russians had been killed.

Russia’s defense ministry denied any involvement.

The Wagner Group is a Russian paramilitary organization, whose contractors have reportedly taken part in various conflicts, including operations in the Syrian Civil War on the side of the Syrian government as well as, from 2014 until 2015, in the War in Donbass in Ukraine.

The founder of the company is alleged to be Dmitriy Utkin, who was born in Kirovograd Oblast (then the Ukrainian Soviet Socialist Republic of the USSR) in 1970.  Wagner was in 2016–2017 believed to have a membership of 1,000–5,000, be registered in Argentina.