Turkish President Erdogan said on 18 May that the purchase of S-400 missile defense systems from Russia was a “done deal” and added that Ankara would also jointly produce S-500 defense systems with Moscow, reported Reuters. US officials considered Turkey’s S-400 purchase “deeply problematic”, saying it would risk Turkey’s partnership in the joint strike fighter F-35 program because it would compromise the jets, made by Lockheed Martin Corp. Erdogan responded that Ankara had carried out technical work and found that such a problem did not exist. He also stated that the US not delivering the F-35s was “not an option.”