Turkey rejected the US offer to transfer to Ukraine the advanced S-400 air defense system it purchased from Russia, Turkish news outlet Lodos Haber reported on Sunday.

Defense Minister Mevlüt Çavuşoğlu said that the US proposal directly affects Turkey’s sovereignty.

As Kyiv Independent reports, the United States has tried to convince countries that operate Russian missiles, such as the S-300 and S-400, to consider providing Ukraine.

The S-400 was developed in post-Soviet Russia and is a more advanced and more powerful version of the S-300, which Ukraine operates widely, but for which interceptor missiles are limited.

The system, which Turkey bought from Russia in 2017, has caused major controversy for Washington, which has imposed sanctions on the Turkish defense industry and excluded it from the F-35 strike fighter program.

Ankara has refused to give in because the weapon has become a point of pride, Reuters reports citing analysts.

Cavusoglu also talked about the F-35 program, saying that Turkey is developing its own fighter jets.