Turkey already proposed to form a joint working group to assess whether the S-400 will pose a threat to NATO or not, said Foreign Minister Mevlut Cavusoglu.
Here's a comparison of some of the strongest weapons from the world of Game of Thrones and their surprising similarities with some weapons in the real world.
Tensions between US and Turkey have worsened in recent months with Turkey set to begin receiving the S-400 Russian surface-to-air missile system, which the US says will jeopardise Ankara's role in the F-35 fighter jet programme.
Remarks by Defense Department spokesman are in contrast to comments made earlier in the day by Senator Jim Inhofe who said a decision was made to stop training the Turkish pilots.
Acting US Defense Secretary Patrick Shanahan's comments came a day after the Pentagon suspended the delivery of equipment related to the F-35 fighter jet to Ankara.
The Pentagon also has started to look at secondary sources to produce parts for F-35s that were being developed in Turkey.
Turkish President Recep Tayyip Erdogan says the pressure against Ankara's purchase of S-400 air defence system is because "Turkey takes action with its own will regarding the regional developments, particularly Syria."
"Turkey does what is necessary for its own interests regarding the S-400 missile defense system, it makes its own decision," Vice President of Turkey Fuat Oktay says.
A report in a major publication alleges that Turkey is considering granting US defence companies access to Russian military know-how as part of its deal to buy the Patriot missile system from the US. Is this even a feasible scenario?
The Pentagon says it will reassess the promised delivery of F-35s to Turkey, but the report doesn't address Ankara's major concerns.
India has requested that the US grant it a waiver for the deal from sanctions prescribed by a US law passed in August 2017 that is intended to punish Russia for its annexation of Crimea and alleged interference in the 2016 US elections.
India is the world's biggest arms importer with Russia and the US as its largest and second largest supplier. Modi and Putin are also set to discuss space cooperation and a possible second Russian-built nuclear power plant in India.
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