The Russian air force has reportedly deployed two prototype stealth fighters to Syria. The deployment comes just two weeks after U.S. and allied warplanes bombarded pro-regime forces, killing potentially scores of apparent Russian mercenaries.

The radar-evading T-50s could complicate the U.S.-led air campaign in the region, said Gen. James Holmes, head of the U.S. Air Force’s Air Combat Command. The United States deployed its own stealth fighters over Syria starting in September 2014.

The T-50s’ arrival underscores Russia’s own ongoing air campaign in Syria, despite Russian President Vladimir Putin’s repeated claims that the Kremlin would wind down its intervention in the war-ravaged country.

Observers on the ground near Latakia in western Syria witnessed the twin-engine, twin-tail T-50s—also known as Su-57s—landing at Khmeimim air base on Feb. 21. The Kremlin did not immediately respond to an email asking for confirmation of the T-50s’ presence in Syria. Russian news agency Interfax claimed it independently confirmed the stealth fighters’ deployment. Video depicting the T-50s’ landing at Khmeimim appears authentic.

Khmeimim has been Russia’s main base in Syria since the Kremlin first deployed troops, planes, and artillery to the country in September 2015 in a bid to prop up the embattled regime of Syrian President Bashar al-Assad.

Russian warplanes flying from the base routinely bomb civilians and anti-regime rebels in addition to occasionally attacking ISIS militants. Russian jets have also interfered with U.S. and allied warplanes on their own missions targeting ISIS. U.S. Defense Secretary James Mattis described the Russian interceptions of coalition planes as “dangerous.” In November 2015, a Turkish fighter jet shot down a Russian bomber that strayed over the Syria-Turkey border.

The supersonic T-50s could intensify the risk to U.S. and allied warplanes. With an angular shape that could help to minimize its radar signature, the T-50 is a rough analogue to the American F-22. The U.S. Air Force includes F-22s on many of the most complex and dangerous air raids in Iraq and Syria.

The twin-engine F-22’s stealth qualities and sophisticated sensors allow it to act as an “aerial quarterback” leading less advanced warplanes, Herbert Carlisle, formerly head of Air Combat Command, said in 2015.

The T-50 could do the same for older Russian and Syrian jets. The deployment “certainly raises the level of complexity the crews have to deal with out there,” Air Combat Command Commander Holmes said, according to a tweet from Aviation Week reporter Lara Seligman.

In an email early Friday, the Combined Joint Task Force acknowledged the presence of the new Russian stealth fighters. “The presence of any new Russian aircraft in the region does not affect coalition operations, nor do we see this as a danger to coalition aircraft. The coalition regularly de-conflicts airspace over Syria with the Russians via a direct de-confliction line. The de-confliction line has been a valuable asset for de-conflicting the congested battlespace in Syria, preventing accidental targeting as well as escalations in tensions or use of force.

“The exchanges between coalition and Russian officials on this de-confliction line have been regular and professional, and thus the process has certainly served its purpose.

“The coalition will continue to support our SDF partners east of the Euphrates River to ensure the lasting defeat of Daesh. The Russians support the Syrian regime in their operations west of the Euphrates.”

It’s unclear how long the Kremlin can sustain the T-50s’ operations. The Russian jet is still in development and lacks many important electronic systems. Warplane-maker Sukhoi has manufactured only about a dozen T-50s. Their reliability is reportedly low. By contrast, the U.S. Air Force possesses more than 180 F-22s. The American jet entered service in 2005 but didn’t fly its first combat mission until 2014.

That Russia has deployed T-50s at all highlights the rising aerial stakes in Syria. On Feb. 7, pro-regime troops—reportedly including Russian mercenaries—attacked U.S.-backed rebel forces in eastern Syria. Coalition warplanes including F-22s bombarded the regime forces, killing an estimated 100 attackers. Potentially scores of Russian mercenaries were reportedly among the dead.

Russian President Vladimir Putin announced his country’s withdrawal from Syria as recently as December. Despite this, Russian warplanes have continued to bomb rebels and civilians. Anti-regime fighters shot down a Russian attack plane on Feb. 3. The pilot ejected but reportedly killed himself to avoid capture. Moscow sent a fresh batch of warplanes to Khmeimim on Feb. 21, including fighters, attack jets, radar planes, and the two T-50s.

The U.S.-led coalition and the Russian military maintain a hotline for “deconflicting” their respective warplanes in order to prevent an aerial clash. But a Russian bomber nearly collided with two American A-10 attack planes in November, according to The New York Times. The same month, a pair of F-22s released infrared flares in an attempt to ward off a Russian jet that was buzzing U.S.-backed forces east of the Euphrates River.

The air over Syria was already dangerous. Now that two Russian stealth fighters have joined the fray, it could become even more dangerous.