Some officials at the State Department and the US Department of Defense blame the US President Donald Trump for failing to put Turkey at a hard ultimatum in the talks, after which Ankara was denied US F-35 fighters, ABC News reported. 

“For months, senior officials, including Vice President Mike Pence, Secretary of State Mike Pompeo and acting Defense Secretary Patrick Shanahan, have warned Turkey against choosing a Russian missile system that it is incompatible with the NATO defense systems. They expressed concerns that Turkey, a NATO ally, would purchase a Russian system over the U.S.-made Patriot system,” the source noted.

In April, the Pentagon suspended deliveries of the F-35 to Turkey. Washington is afraid that the simultaneous purchase of US fighter jets and the S-400s will result in Russia being able to gain access to secret technologies.