Turkey won't step back on S-400, senior official says amid F-35 negotiations with Washington

Presidential spokesperson İbrahim Kalın said Friday Turkish and U.S. officials have started negotiations on the F-35 fighter jets and Turkey's acquisition of Russian S-400 missile defense systems.

"The officials have started working as part of a mechanism to evaluate the impact of Russian S-400s on F-35 jets," Kalın told Turkish state news broadcaster, TRT Haber.

"It is out of the question for Turkey to take a step back from its purchase of the [S-400] system," the spokesman said.

After its long efforts to purchase U.S.-made Patriot missiles failed, Turkey acquired two batteries of the Russian-made S-400 air missile defense systems as part of a $2.5-billion deal inked in December 2017.

The deal caused tension between Ankara and Washington as the latter even threatened sanctions on Turkey. U.S. President Donald Trump, however, has said he was not considering punitive measures.

The U.S. eventually suspended Turkey, a fellow NATO member, from its F-35 stealth fighter program, in which Turkey is a major manufacturer and buyer.

Washington argues the S-400 would be incompatible with NATO systems and expose the F-35 to possible Russian subterfuge.

Turkey, however, emphasized the S-400 would not be integrated into the NATO systems and would not pose a threat to the alliance, refusing to step back from the deal with Russia.