ORLANDO -- Russia's Su-57 stealth fighter, like China's Chengdu J-20, has become a bogeyman to the United States in the competition to possess the best fifth-generation fighters in the world.

But has the Su-57 made its debut in Syria? The Pentagon, won't confirm it.

"The addition of fifth-generation fighters into Syria would certainly not be in keeping with Russia's announced force drawdown," said Eric Pahon, Defense Department spokesman.

"We do not consider these jets to be a threat to our operations in Syria, and will continue to deconflict operations as necessary," Pahon said in a statement Thursday.

"The coalition remains focused on the enduring defeat" of the Islamic State, he added. "We call on all parties, however, to remain focused on defeating ISIS, de-escalating and resolving the Syrian conflict, and protecting innocent civilians."

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On Wednesday, videos of two Su-57s began circulating on social media, showing the jets landing at Khmeimim air base in Western Syria.

The Sukhoi-made jet, known also as the T-50 or PAK-FA, is a single-seat, twin-engine multirole fighter intended to go up against the Air Force's F-22 Raptor and F-35 Joint Strike Fighter.

Russia has been swift to procure and test the fifth-generation-like aircraft in light of the F-35's initial operating capability, which it achieved in 2016.

Russia's new Sukhoi lies "somewhere between the F-22 and F-35," according to Doug Barrie, senior air analyst at the International Institute for Strategic Studies.

Barrie told Air Force Times in 2016 that even though the T-50 has the sophisticated agility of a future fighter, it will not be as advanced as the most capable U.S. platforms.

On Thursday, Gen Mike Holmes, Air Combat Command commander, said he was unaware of reports surrounding the Su-57 in Syria, but added the situation in Syria grows more complex by the day.

"It's one thing to do the counter-air mission with a long lookout in front of you, it's different to do when everyone's tightly packed in there," Holmes said during a roundtable discussion with reporters here at the Air Force Association's Air Warfare symposium.

"So our guys will continue to prep for the scenarios that they're deployed to face," Holmes said.

When asked whether it was concerning to have another stealth aircraft in the mix, Holmes emphasized the tough environment and constricted air space in the region.

"Certainly, the higher the complexity and the higher the technology ... it raises the level of complexity for the crews to deal with," the commander said.

Russia first deployed forces and aircraft to Syria in 2015, changing the dynamic as U.S. and coalition troops began their air campaign against the Islamic State a year earlier. Russian President Vladimir Putin said a withdrawal of their bulk of troops would begin in 2017.

It's not surprising the Russian air force may want to step up its tactics and procedures in a war environment.

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.

"In the skies over Syria, it's really just been a treasure trove for [the Russians] to see how we operate," Lt. Gen. VeraLinn "Dash" Jamieson told congressional staffers and reporters during an Air Force Association briefing in Washington, D.C.

"Our adversaries are watching us, they're learning from us," she said at the time.

That same month two F-22 fighters intercepted two Russian Su-25 fighter jets, conducting multiple maneuvers, firing warning flares and, in one instance, aggressively flying to avoid colliding with one another.

A Russian Su-35 multi-role fighter was also involved.

An F-22 ended up trailing the Su-35 after it flew across the river into territory deemed unsafe to coalition aircraft.

-- Oriana Pawlyk can be reached at oriana.pawlyk@military.com. Follow her on Twitter at @oriana0214.