The Washington Post report will raise questions about how much the Russian government knew ahead of the attack, whether it was officially sanction and at what level it would have been authorised.
The clash in Deir al-Zor Province appears to be the first time that USA forces engaged directly with Russians in Syria, where Russian forces are backing President Bashar al-Assad's government in a devastating seven-year civil war. It said the Su-57 is still not fully operational and that Russian Federation is likely going to use Syria as a "playground" for testing the jet. The coalition remains focused on the enduring defeat of ISIS. Official Moscow denies its involvement in the campaign in Syria.More news: Cardinals' compensatory picks worth more than any other team's haul
The combat plane was developed to overpower the F-22 Raptor stealth fighter planes of the US. "In a state that has huge budget problems and a massive military bill", the Su-57 functions as a prestige item, said Bronk.
Russian President Vladimir Putin said a withdrawal of most of their troops would begin a year ago.
USA officials estimate 500 pro-regime troops were involved in the attack on a compound where both U.S. troops and their allies in the Syrian Democratic Forces were stationed.
Last month, the Air Force's deputy chief of staff for intelligence, surveillance and reconnaissance said that the Russian air force has been focusing on intently watching how fourth- and fifth-generation aircraft perform within the Syrian airspace.More news: Korean 'Iron Man' Kim wins historic Olympic skeleton gold
It's a deployment that could potentially increase the level of danger to U.S. forces operating inside the country.
When the US soldiers called in airstrikes to defend themselves, they had no idea they would end up killing dozens Russian civilians.
Moscow appears to have deployed an advanced new stealth fighter to its airbase in Syria, reports in Russian news media and online videos of the aircraft indicated on Thursday, in what analysts say could be a risky attempt to gain publicity and operational experience for the jet in one of the world's most tangled conflicts.
The revelation that Russian mercenaries were killed stoked fears of an outright hostile confrontation between Russians and US forces.More news: School safety officer thwarts alleged school shooting plot