ANKARA (Reuters) - Turkey will not step back from its purchase of Russian S-400 missile defense systems, a senior aide to President Tayyip Erdogan said on Friday, two days after talks with U.S. President Donald Trump.

Erdogan and Trump held talks in Washington on Wednesday to overcome mounting differences between the NATO allies, ranging from looming sanctions threats over Turkey’s purchase of S-400s and Syria policy. Washington says the S-400s pose a threat to its F-35 jets and has suspended Turkey’s involvement in the jet program.

Speaking to broadcaster TRT Haber, Ibrahim Kalin said that Turkish and U.S. officials had begun work as part of a mechanism to evaluate the impact of the S-400s on the F-35 jets on Friday.