NATO Secretary-General Jens Stoltenberg has expressed concern about the potential consequences of Turkey's plans to purchase Russian air-defense missile systems.
Stoltenberg was speaking on May 6 during a joint press conference with Turkish Foreign Minister Mevlut Cavusoglu in Ankara, amid a widening rift between Washington and Ankara over Turkey's deal to buy S-400 surface-to-air missile systems.
The United States has demanded that NATO ally Turkey call off its deal with Russia, saying that the S-400 missiles are incompatible with NATO systems and are seen as a threat to U.S.-made F-35 fighter jets.
Washington has said it could also withdraw an offer to sell Ankara the U.S. equivalent -- the Patriot anti-missile system – and warned that Turkey risks being ejected from the F-35 fighter-jet program.
Turkey is a member of the consortium involved in the production of the jet and a buyer.
In Ankara, Stoltenberg said that every NATO ally makes its own decisions on the kind of military equipment it buys.
"But at same time I am concerned about the potential consequences as the United States has made it clear that they will impose sanctions" if Turkey goes through with the deal, the NATO chief added.
"What is important for NATO is interoperability -- that systems can work together," he said.
Turkish President Recep Tayyip Erdogan has repeatedly said that his country won't withdraw from the S-400 deal, which according to Russian media involved four S-400 units for a price of $2.5 billion.
Speaking alongside Stoltenberg on May 6, Erdogan said that he disapproved of attempts to provoke debate on issues like the S-400 that "are within the sovereign rights of our country."
Turkey's developing ties "with other countries and regions are not an alternative to each other," he said, adding that instead they complemented each other.
Turkey's relations with Russia have historically been tense, but Moscow and Ankara have established strong economic ties since the end of the Cold War.
Under Erdogan and Russian President Vladimir Putin, the countries have moved closer in recent years amid severe tension between Russia and the West and strains in Turkey's ties with the United States and European Union.