Since its receipt by Turkey earlier this month, the Russian S-400 missile system has triggered a strong response by its fellow NATO member the United States.
Training by Turkish pilots on F-35 fighter jets has come to a faster-than-expected halt at a U.S. air base in Arizona.
U.S. Vice President Mike Pence warned Turkey on Wednesday against buying a Russian S-400 anti-missile system.
Turkey and Russia on Friday signed an accord for Moscow to supply Ankara with S-400 surface-to-air missile batteries.
The Kremlin said on Monday that talks with Saudi Arabia over Moscow supplying Riyadh with advanced S-400 air defence missile systems has gone well from Russia's