MOSCOW (AP) — Turkish President Recep Tayyip Erdogan and Russian President Vladimir have underscored their close links as they opened a major Russian air show.

Erdogan was Putin's guest of honor on Tuesday at the opening of the MAKS aviation show outside Moscow and even got to peek inside the cockpit of Russia's newest fighter jet, the Su-57, which was unveiled at the show.

Turkey's military ties with Russia have been a cause of concern for NATO and the United States after Ankara went ahead with its purchase of Russia's S-400 missile defense system.

The United States says the Russian system is incompatible with NATO and poses a threat to the U.S.-led F-35 fighter jet program. It suspended Turkey's participation in the F-35 program and has also threatened sanctions against Ankara.