Several Russian mercenaries working in Syria were killed by U.S. airstrikes last week, according to reports in Russia and in the West. However, Washington and Moscow have yet to confirm the deaths amid reports the number of Russian casualties could be far higher.

Russian media reported that Russian military contractors died in the airstrikes launched on Feb. 7, when pro-regime forces advanced on oil fields in the eastern province of Deir ez-Zour and were targeted by U.S. air and artillery strikes.

Western outlets including the BBC and CNN have also confirmed that Russian contractors were killed, citing their families, friends and associates. Both outlets named two of the dead as Kirill Ananyev, from Moscow, and Vladimir Loginov, a member of a Cossack group of ultranationalists from Kaliningrad.

“Vladimir died for the Fatherland, the Cossacks and the Orthodox faith!” read a statement from Loginov’s group, the Baltic Cossack District, which said he died on Feb. 7 while “heroically defending our Fatherland in its far reaches from crazy barbarians.”

CNN and BBC reported that both men worked for the Russian private military contractor Wagner Group, a Hong Kong-registered company that has sent many ex-servicemen to Syria to fight in support of Russian and regime forces. The group is under U.S. sanctions for its support of separatists in eastern Ukraine.

But the U.S. and Russia have remained coy about the reports, which would mark the first known occasion that U.S. airstrikes have killed Russians in the Syrian conflict.

President Vladimir Putin's spokesman Dmitry Peskov told reporters Tuesday he knew nothing about Russian mercenaries being killed, and only knew about Russians deployed within the armed forces.

“We don’t have information about other Russians who might be in Syria,” he said.

Russian authorities do not publicly comment on the subject of private military contractors in Syria. Russian media outlets have reported that thousands of Russians have been deployed in the conflict as private contractors, helping the Kremlin to keep the official death toll from its involvement low.

U.S. Defense Secretary Jim Mattis told reporters Tuesday that he had seen the reports, but had no information about Russian contractors in the area of the strike.

Russian media outlets have claimed the Russian deaths in the incident could be much higher than the few contractors reported so far, but those reports have not been confirmed.

The incident could prove embarrassing for both sides, which have maintained a deconfliction line to avoid unintentionally striking each other’s forces in the chaos of the battlefield and accidentally setting off a wider confrontation.

The two sides have given differing accounts of last week’s incident, which reportedly left about 100 pro-regime fighters dead.

U.S. officials say that they launched strikes in self-defense after about 300 pro-regime fighters launched a coordinated attack on the positions of the U.S.-backed Syrian Democratic Forces near the town of Khursham, Deir ez-Zour, including an SDF base where American advisers were stationed.

They say they were in contact with Russian counterparts before, during and after the attack, and that Russian officials assured them they would not strike coalition forces.

Russia says that pro-regime troops were hit as they launched an operation against ISIS. It says that none of its forces were involved in the incident, and that the pro-regime units involved had not coordinated their actions with Russian military advisers.

Cover image: Smoke billows from the eastern Syrian city of Deir ez-Zou during an operation by Syrian government forces against Islamic State (IS) group jihadists on November 2, 2017. (STRINGER/AFP/Getty Images)