The head of NATO is in Turkey trying to smooth tensions between two key allies. The United States strongly opposes Turkey's purchase of a Russian missile defense system. CGTN's Michal Bardavid has more.

NATO Secretary General Jens Stoltenberg attended the twenty-fifth anniversary meeting of the Mediterranean Dialogue held in Ankara. He then held talks with Turkish President Recep Tayyip Erdogan and the Turkish Foreign Minister Mevlut Cavusoglu.

Speaking at the forum, Stoltenberg emphasized Turkey's value to the alliance and stated that NATO stood with Turkey as it faces serious security challenges. During Stoltenberg's talks with Turkish officials, Ankara's controversial agreement with Moscow to buy an S-400 missile defense system was top of the agenda.

MICHAL BARDAVID ISTANBUL "The U.S. has been urging TURKEY to withdraw from its deal with Russia, stating that the move would compromise the F-35 Joint Strike Fighter program. Washington has already taken concrete steps in reaction to this deal as it halted the delivery of equipment related to the F-35 fighter aircraft to Turkey. U.S. officials have also threatened Turkey stating that going forward with the deal meant risking expulsion from the F-35 program.

Stoltenberg expressed his concerns over the agreement.

JENS STOLTENBERG NATO SECRETARY GENERAL "I am concerned about the potential consequences of the decision to buy S-400 because the U.S. has made it clear that they will impose sanctions. So this is an issue which has been addressed several times in my meetings in Washington and Ankara and I hope that it is possible to find a solution because we have to avoid conditions when one NATO ally is imposing sanctions against another."

But Turkey remains defiant. Officials have repeatedly stated Ankara could not abandon the Russia deal and that payments had already been made. Washington says it wants Turkey to choose between Russia and NATO but Turkey's President Recep Tayyip Erdogan stressed the decision was one of national security and not a political one.

RECEP TAYYIP ERDOGAN TURKISH PRESIDENT "The relations Turkey has established with different countries and regions are not alternatives for one another but complementary to each other. We don't appreciate efforts to revive disputes over issues such as the S-400 missiles. It is a matter of our country's sovereign rights."

Analysts say the S-400 agreement with Moscow is expected to damage Turkey- U.S. ties and likely hurt the economy if Washington imposes sanctions as promised. Nevertheless, it seems Ankara is determined to follow through with its deal with Russia. Michal Bardavid, CGTN, Istanbul.