Russian news service Komsomolskaya Pravda reports anonymous sources as saying: "They simply rolled us ... Artillery first, then helicopters .. Surely they saw that we were preparing for an assault on our beachhead on the left bank ... As a result, almost everyone from the 5th assault detachment died, they were burnt together with their hardware."
At the heart of the scenario is an assault last week by some 500 pro-Syrian President Assad regime troops on an Deir al-Zor encampment of US-backed Syrian Democratic Force (SDF) rebels.
US Defense Secretary General James Mattis said over the weekend he did not understand the circumstances behind the confusion — as well established lines of communication and procedures had been used to prevent just such an incident.
"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.
"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."
But Russian media has been filled with reports of dead nationals — killed by US fire in Syria's Deir el-Zour province.
Russian casualties
"Vladimir died for the Fatherland, the Cossacks and the Orthodox faith," the ultranationalist Baltic Cossack District said in a statement. He died in an "unequal battle ... heroically defending our Fatherland in its far reaches from crazy barbarians."
They and Russian media were referring to Vladimir Loginov, 51. He was a Cossack who had also fought in Ukraine.
Exactly how many Russians died in last week's incident is uncertain. Reports vary from "several" to more than 100.
Russian social media is alive with reports and rumours.
Several upset friends and family members have also approached Russian news outlets to report that military "contractors" belonging to the Moscow-based firm Wagner are among the dead.
Wagner is a "security services" company subject to international sanctions because of its actions in Ukraine. It is believed to have several hundred 'contractors' fighting in Syria.
One of them was Kirill Ananyev, a friend claimed, who had been killed when "American artillery smashed the Wagner unit" last week.
The Kremlin has so far refused to confirm or deny the reports.
But Russian opposition presidential candidate Grigory Yavlinsky took to Twitter to declare: "If massive Russian casualties took place, then relevant officials ... must announce this to the country and find out who is responsible."
Confused circumstances
It's a scenario that has threatened to unfold for some time. US and Russian forces have been facing each other from opposite sides of the Syrian Civil War almost since it began in 2011.
Only the designation of the Russian advisers as "mercenaries" or "contractors" provides an out from this escalating into a full-blown diplomatic crisis.
These so-called Russian "mercenaries" were playing a similar role to the US military's official "advisers" embedded among the SDF forces: offering training and tactical advice.
But the Russian contractors also operated as independent, cohesive fighting units. At least two of these appear to have recently taken up a much more frontline role.
If they had openly been Russian frontline troops, it would be another matter entirely.
Syrian regime sources say the attack was an unauthorised effort by local loyalist "militia" forces against Islamic State.
But Operation Inherent Resolve spokesmen have contradicted that claim, saying they had observed a steady build-up of tanks, equipment and supplies in the week leading up to the clash.
CIA Director Mike Pompeo was asked about the presence of Russian contractors at a Senate Intelligence Committee hearing yesterday. He replied that the Pentagon had the details, but added "we have seen in multiple instances foreign forces using mercenaries in battles that will begin to approach the United States."
Surprise attack
The attack in Deir el-Zour province took place on the eastern banks of the Euphrates river, which comes under the jurisdiction of US-led Coalition combat air operations in the fight against Islamic State. Russian and Syrian warplanes have free-reign over the western side.
On Wednesday, pro-Syrian ground forces — including a number of Russian-built T-55 and T-72 tanks, howitzers, mobile rocket launchers and 500 troops — surged to within 500m of the rebel enclave.
Their objective may have been to seize control of the area's rich oilfields.
But the US response was immediate and intense — a three hour onslaught by AC-130 Hercules gunships, F-15 strike fighters, F-22 stealth fighters, Apache helicopters and US Marine artillery.
"There was incoming fire, and we were with the SDF, hunkered down, not provoking, and a force is massing and coming at us, so we're going to defend ourselves," the head of US Air Forces Central Command, General Jeffrey Harrigian, said overnight.
US Pentagon officials earlier said the attack was "unprovoked".
The heavy response was in part due to the presence of a detachment of US military "advisers" within the largely Kurdish rebel stronghold. The Pentagon states this was a well-known fact to Russian and Syrian forces. Therefore the attack was not likely to have been a "mistake".
Rebels claim some 100 pro-Syrian troops were killed. Syria conceded the loss of about a 'dozen" fighters.
But Syrian sources reported a mass funeral in the pro — Syrian regime town of al-Suqaylabiyah among a local unit dubbed "ISIS Hunters". The militia had lost about 30 members.
General Harrigian said he would not speculate on who was controlling the attacking force, or who it was comprised of.
"I think it's important to recognize that it's not as simple to sort out exactly who everybody is down there," he told reporters at the Pentagon. "Right now 'pro-regime force' is probably the best definition we have to describe who they were. It's really just not helpful for me to speculate … there's a fair number of groups involved with this and it's always difficult to precisely sort that out."
