Russia has sent its most advanced fighter jet to Syria, a US military official tells CNN. It's a deployment that could potentially increase the level of danger to US forces operating inside the country.
The Su-57 is Moscow's most sophisticated military jet and comes equipped with stealth capability.
Russia's move comes despite a promise from President Vladimir Putin to scale back Russia's military commitment in the country.
"The addition of fifth generation fighters into Syria would certainly not be in keeping with Russia's announced force drawdown," Pentagon spokesman Maj. Adrian Rankine-Galloway told CNN.
The deployment comes on the heels of the United States employing its most advanced fighter jet, the F-22 stealth fighter, in strikes against targets in Syria after a February 7 attack against US-backed forces by pro-Syrian government troops that also included Russian mercenaries.
US officials estimate 500 pro-regime troops were involved in the attack on a compound where both US troops and their allies in the Syrian Democratic Forces were stationed.
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.
The US jets fired warning flares during the intercept of the two Russian Su-25 close air support jets according to US officials after they crossed the de-confliction line multiple times.
"We do not consider these jets to be a threat to our operations in Syria, and will continue to deconflict operations as necessary," Rankine-Galloway said.
The Russians have based the jet at their Khmeimim airbase in Syria according to the US military official.
It is unclear if their deployment is a response to US deploying F-22s to the region or if Russia is using the Syrian theater as an opportunity to test the new aircraft.
The Su-57 is the first Russian military fighter to use stealth technology and is designed for aerial combat and ground attack operations.
According to the Russian Defense Ministry, the jets were scheduled to be introduced to Russian forces in 2018, and a total of 12 aircraft were initially ordered by the Russian government.