The United Arab Emirates has agreed to purchase a batch of advanced Sukhoi Su-35 Flanker-E fighters from Russia.

The Flanker-E is the most capable operational Russian combat aircraft would add to the UAE’s already formidable fleet of Lockheed Martin F-16E/F Block 60 Fighting Falcons. More troubling for the United States, the deal is an indication that the UAE—a long-time U.S. ally—is drifting into Moscow’s orbit.

"We signed an agreement of intent for the purchase of the Su-35,” Rostec chief Sergei Chemezov told the Moscow-based TASS news agency.

Chemezov did not offer any details about how many Su-35s the UAE has ordered or when the aircraft would be delivered. The UAE also recently signed an agreement to co-develop a new fifth-generation fighter with Russia.

The Su-35 is a formidable fighter. As an air-superiority fighter, its major advantages are its combination of high altitude capability and blistering speed—which allow the fighter to impart the maximum possible amount of launch energy to its arsenal of long-range air-to-air missiles. The Su-35 would be launching its weapons from high supersonic speeds around Mach 1.5 at altitudes greater than 45,000 ft.

The Su-35 builds on the already potent Flanker airframe, which in many respects already exceeded the aerodynamic performance of the Boeing F-15 Eagle. The Su-35 adds a lighter airframe, three-dimensional thrust vectoring, advanced avionics and a powerful jamming capability.

“Large powerful engines, the ability to supercruise for a long time and very good avionics make this a tough platform on paper,” one highly experienced F-22 pilot told me some time ago. “It's considered a fourth gen plus-plus, as in it has more inherent capability on the aircraft. It possesses a passive [electronically-scanned array and it] has a big off boresight capability and a very good jamming suite.”

The Su-35 also carries a potent infrared search and track capability that could pose a problem for Western fighters. “It also has non-EM [electro-magnetic] sensors to help it detect other aircraft, which could be useful in long-range detection,” a Super Hornet pilot told me.

Overall, the Su-35 is an extremely formidable machine—and the Russians are starting to have success with selling it abroad. The Chinese have already bought 24 examples while Indonesia and Brazil are apparently interested in purchasing some number of aircraft too.

Dave Majumdar is the defense editor for The National Interest. You can follow him on Twitter: @davemajumdar.