If true, the battle may mark the deadliest encounter between the Cold War rivals in decades.
While the Kremlin has declined to comment, and no independent party has yet verified the reports, US and Russian aligned forces have fought on opposite sides of the Syrian conflict and in close proximity for years.
If the U.S. did kill Russian military contractors, it falls short of killing official Russian servicemembers, which could escalate into a larger war.
But the loss of Russians in Syria may still blacken the image of the Kremlin's intervention in the six-year civil war, which it portrays as peace-keeping and inexpensive.
Russian media said Russian private contractors and pro-government forces advanced on oil fields in the eastern Deir el-Zour province and were targeted by the United States.
“Pro-regime forces initiated what appeared to be a coordinated attack on Syrian Democratic Forces east of the Euphrates river,” Pentagon spokesperson Dana White said in a statement, referring to the SDF, which the US has trained, equipped, and backed for years.
The river acts as a border between the coalition and Russian and Syrian forces, and the Pentagon also described the SDF location as well-known, and that therefore the attack wasn't a mistake.
Syrian regime forces launched a coordinated attack that included about 500 regime troops, 122mm howitzers, tanks and multiple launch rocket systems on the U.S.-backed SDF headquarters in Deir al-Zor province approximately five miles east of the Euphrates River.
President Donald Trump hit back at House Speaker Nancy Pelosi on Thursday with a letter saying he was "sorry to inform" her that her trip to Brussels, Belgium and Afghanistan would be canceled due to the government shutdown, just one day after Pelosi proposed cancelling the State of the Union address for similar reasons.
Marine Corps drill instructor R. Lee Ermey in his iconic role in 'Full Metal Jacket' (Warner Bros.)
Retired Marine Staff Sgt. R. Lee Ermey, the legendary Marine drill instructor turned iconic Full Metal Jacket actor who died last year, will be formally laid to rest with full military honors at Arlington National Cemetery at 10 a.m. on Jan. 18, according to the cemetery's web site.