The U.S. has issued a series of warnings to Turkey over its purchase of the S-400.
"As for the S-400, the deal is done, and it is out of the question to go backwards.and perhaps after the S-400, we will move onto the S-500", Erdogan told Kanal-24 television on March 6.
North Atlantic Treaty Organisation member Turkey has repeatedly said it is committed to buying the Russian missile defence system, despite warnings from the United States that the S-400s can not be integrated into the North Atlantic Treaty Organisation air defence system.
The US fears the Russian system could be used to collect intelligence on the jet's stealth capabilities, but Erdogan said "we've researched it, there is no such thing".
Russian S-400 missile air defence systems on display during a parade marking the 73rd anniversary of the victory over Nazi Germany in World War Two, at Red Square in Moscow, May 9, 2018. "We've agreed with the Russians, we will go into joint production".
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"We signed a deal with Russian Federation for the purchase of S-400, and will start co-production".
Summers said those consequences would encompass losing USA military sales to Turkey, including the long-awaited sale of the United States' new F-35 Joint Strike Fighter jet. "In total, F-35 industrial opportunities for Turkish companies are expected to reach $12 billion", according to information posted by Lockheed Martin, the jet's manufacturer.
"I would just underscore the fact that this is a... huge decision for Turkey", he said regarding the F-35, adding: "But also to FMS (foreign military sales) and other systems we sell to Turkey as well". Erdogan considers these forces inextricably linked with Kurdish terrorist groups operating domestically and in Syria, and the Turkish military has already begun targeting elements of these units to ensure they don't maintain control along the Turkish border once the Islamic State group is defeated. He has previously criticized Ankara for not doing more in the fight against the Islamic State group, and was particularly vocal about Turkey's detention of an American pastor released last year.
"If Turkey takes the S-400s there will be grave consequences", acting chief Pentagon spokesperson Charles Summers told reporters Friday, saying it would undermine America's military relationship with Ankara. "God willing, we will receive the first system in July".