Russian officials distanced themselves from the reports initially, pointing out that the people killed were not Russian servicemen and that their nationals can be found in many places around the world.
But if they are there, the Pentagon said it is not happy.
In December, two F-22's intercepted two Russian aircraft that had flown east of the "de-confliction line" that is supposed to separate Russian and US-led coalition aircraft operating over Syria.
Russian Federation often uses Syria as a showroom for new hardware, and likely wants to market the Su-57, which is still a prototype, as having been combat tested. "We call on all parties, however, to remain focused on defeating ISIS, de-escalating and resolving the Syrian conflict, and protecting innocent civilians".
The Russian Su-57 is a fifth-generation aircraft that has been in development for years and has been deployed at an active combat theatre for the first time. With the apparent deployment of the Su-57, Russia may be teaching its best pilots in its newest plane how to stalk and fight F-22s, which would rely on stealth as their major advantage in combat with more maneuverable Russian jets.
Former Google employee files lawsuit claiming he was sacked for opposing discrimination
Former Google employee Tim Chevalier has reportedly sued the search giant for firing him over his internal pro-diversity posts. Google, on its part, says that employees should be communicated in a manner that complies with the company's policies.
"In the skies over Syria, it's really just been a treasure trove for (the Russians) to see how we operate", Lt. Gen. VeraLinn Jamieson said during a Mitchell Institute for Aerospace Studies briefing in the Capitol. The first stage of state trials has been concluded.
Had Moscow simply agreed to what Berlin is proposing without getting the radical militants out of the way, the Russian commentator suggests, any United Nations peacekeepers would have been opposed by its own militants in the Donbas.
The deployment of the jets comes two months after Russian President Vladimir Putin ordered a partial withdrawal of Russian troops from the war-torn country. US officials believe the soldiers-for-hire were attacking a gas installation right next to the base controlled by USA special operations forces and their Kurdish allies.
Igor Eidman, a Russian commentator for Deutsche Welle, suggests that may be the case with the recent deaths of Russian mercenaries in Syria.