NATO Secretary-General Jens Stoltenberg is visiting Ankara amid a widening rift between alliance members Turkey and the United States over Turkish plans to purchase a Russian missile system.
Stoltenberg said he encouraged the "ongoing dialogue" between Turkey and the United States over Russia's S-400 surface-to-air missile system, in an interview on May 5 with Turkish state news agency Anadolu.
But he also welcomed "talks about Turkey's possible acquisition of a U.S. Patriot missile system," in an indication that he could get Turkey's leadership to reconsider its stance on the S-400.
President Recep Tayyip Erdogan has repeatedly said Turkey won't withdraw from the S-400 deal, despite mounting pressure from Washington and NATO.
Washington has also 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.
"Decisions about military procurement are for nations to make," Stoltenberg told Anadolu when asked about the S-400. "But...interoperability of our armed forces is fundamental to NATO for the conduct of our operations and missions."