Russia has deployed two of its most advanced and sophisticated stealth jets in Syria, in a power move likely to draw US criticism, as the Pentagon expressed its alarm earlier this month over recent close calls with Russian planes in the war-torn country.

Satellite images published by Israeli company ImageSat International on Friday showed the fifth-generation jets — two Su-57 fighter aircraft — at Russia’s airbase in the coastal Syrian city of Latakia. A source in the defense ministry confirmed to RBK news agency this week that the two planes were sent to the Hmeimim base “for a test in real conditions.” The jets are said to be a potential rival to the US’s Lockheed Martin F-22 Raptor, which American forces use in patrols over Syria.

The deployment came with a covert warning to Israel by a Russian official, who said that the presence of the Su-57s will doubtlessly send a political message, serving as a deterrent “for aircraft from neighboring states, which periodically fly into Syrian airspace uninvited.”

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Speaking to the Russian news network Sputnik, Vladimir Gutenov, chairman of the Military Industry Committee in the Russian parliament, said on Friday that the jets “need to be tested in combat conditions, in conditions of [enemy] resistance.”

New photos showing the additional 2 Russian Su-57 escorted by #RuAF Su-30SM

landing today in the Russian Air Force base in #Khmemeim #Syria And a photo for the RuAF A-50U #41 Red landing at the AB after a combat mission. pic.twitter.com/FIV8xcpeuJ — Wael Al Hussaini (@WaelAlHussaini) February 23, 2018

Since 2015, when Moscow began lending air support to the regime of President Bashar Assad and carrying out airstrikes across the country, Russia and Israel have maintained an understanding aimed at avoiding accidental confrontations between the two countries in Syrian airspace. The countries use a “hotline” to communicate on security coordination.

Earlier this month, that mechanism was tested when Israel shot down an Iranian drone that has entered Israeli airspace, leading to large-scale Israeli strikes in Syria and heavy Syrian counter-fire. An Israeli F-16 fighter jet subsequently crashed under fire from Syrian air defenses in a severe increase in tensions.

In the wake of the attacks, Israel had appealed to Russia to intervene and prevent further escalation, and conveyed to Moscow that the events were proof that its warnings of Iranian entrenchment and growing boldness in Syria were merited. Russia has subsequently called on Israel and Iran to “show restraint.”

Israel has, over the years, carried out a number of airstrikes in Syria, reportedly targeting the Iranian-backed Hezbollah cells and weapons shipments.

Meanwhile, Russia has routinely used Syria as a testing ground for weapons and latest military technology. A Russian official said earlier this week that Moscow has tested over 200 weapons during the conflict, now in its eighth year.

“As we helped the brotherly Syrian people, we tested over 200 new types of weapons,” said Vladimir Shamanov on Thursday. Shamanov is a former commander of Russia’s airborne troops, who now serves as head of the Russian Duma’s defense committee.

Earlier this month, the Pentagon voiced growing concern that the risky flying of Russian pilots in Syria could lead to a mishap — or even the nightmare scenario of a US jet shooting down a Russian warplane.

Defense officials highlighted several recent close calls with Russian planes, including one on February 14, when a pair of US F-22s intercepted two Russian jets over a part of Syria in which the Pentagon says they are not meant to be operating.