(CNN) - Several Russians who had been hired as paramilitary contractors were among more than 100 men killed in US airstrikes in northern Syria last week, according to their friends and families.
The men were working for a private Russian company called Wagner, which has sent hundreds of private contractors to Syria to help both the Russian military and pro-regime forces, according to people who knew them.
One of the dead was 51-year-old Vladimir Loginov. Like many contractors who have gone to Syria, he was a member of a Cossack group of ultranationalists who have also fought in eastern Ukraine.
The Baltic Cossack District said in a statement that Loginov was a Russian citizen who was killed in an "unequal battle" while "heroically defending our Fatherland in its far reaches from crazy barbarians" when he died on February 7.
"Vladimir died for the Fatherland, the Cossacks and the Orthodox faith!" the group said.
CNN has been able to confirm the names of at least three other Russians who were killed supporting pro-regime militia in a large-scale attack that day. According to US officials, a force of some 500 crossed the Euphrates River near Deir Ezzor, along with tanks, missile launchers and artillery.
Their target was a base of the US-backed Syrian Democratic Forces, which came under artillery fire. A US detachment was at the base. The US responded with airstrikes that killed about 100 of the attackers, according to the coalition, with the rest retreating.
One of the Russians killed was Kirill Ananyev, according to his friend Aleksandr Averin. He said Ananyev had been in Syria "a little over a year" and that he had been killed when "American artillery smashed the Wagner unit."
Averin said he believed that casualties among the Russians present amounted to more than "the typical Wagner loss of just a couple of people."
Most of the casualties, according to local sources, appear to have been members of a pro-regime Christian militia from the town of al Suqaylabiyah. Earlier this week, there was a mass funeral for about 30 members of the militia -- which often describes itself as the "ISIS hunters" -- in the town.
Two sons of a prominent tribal leader were also killed, according to social-media posts from Syria.
Wagner, which is under US sanctions for supporting separatists in eastern Ukraine, has several hundred contractors in Syria, according to analysts. In December, Nikolay Tikhonivch from Tomsk was killed "in the area of the Hama-Idlib highway," according to a social-media post from the "Rus" patriotic group to which he belonged.
Two Wagner contractors who were captured by ISIS in November have not been heard from since appearing in a video soon after being seized. They are believed to be dead, according to Russian officials.
Wagner is registered in Hong Kong and has no public offices in Russia and no contact details. Efforts by CNN to reach the company were unsuccessful.
US Defense Secretary General James Mattis has said that the US was puzzled by the attack near Deir Ezzor last week.
"The Russians told us they had no forces there initially. I think that's still the case but we don't have full clarity on what the regime forces are doing there," Mattis said over the weekend.
The US and Russian militaries have de-confliction channels to prevent accidental clashes in Syria and alert each other to movements.
"The Russians professed that they were not aware when we called them about that force that had crossed [the Euphrates]. As it came closer they were notified when the firing began," Mattis said.
Neither the Kremlin nor the Russian Ministry of Defense talks about private military contractors in Syria. Kremlin spokesman Dmitry Peskov said Tuesday: "We have no information regarding other Russians who may be in Syria."
It is very difficult to estimate the number of Russians working as military contractors in Syria and the number who are killed.
For days, rumors have swept through Russian social media that far more than a handful of Wagner contractors were killed in last week's engagement. On Monday liberal presidential candidate Grigory Yavlinsky said: "If massive Russian casualties took place, then relevant officials ... must announce this to the country and find out who is responsible."
Kremlin spokesman Peskov rejected Yavlinsky's assertion, saying: "I don't think Yavlinsky has more reliable sources than this."
There were many Russian citizens overseas, Peskov said, "and it's very hard to have detailed information."