Chairman of the U.S. Joint Chiefs of Staff Gen. Joseph Dunford said at the annual Aspen Security Forum in Colorado that Turkey's possible purchase of Russian S-400 missile defense systems is "concerning."
"Turkey has not completed a deal with Russia yet. If they would, it would be concerning," Dunford said in an interview with the NBC television station at the forum.
Last week, Defense Minister Fikri Işık, who was appointed deputy prime minister in the recent Cabinet shakeup, said that the technical aspects of the ongoing deal with Moscow for the S-400 systems have been completed and negotiations had reached the final phase.
Reports claim that Turkey agreed to pay $2.5 billion to Russia for four batteries of S-400 missile systems.
The S-400 was developed as an upgraded version of the S-300 series of surface-to-air missile systems.
The system entered service in April 2007, and the first S-400 was deployed in combat in August 2007 with Russia's incursion into Georgia.
Despite some objections regarding the compatibility of a Russian-made missile system with those NATO countries use, Ankara reached the deal, saying that S-400 batteries will not be aligned with the military bloc's systems.
Responding to Dunford's comments, President Recep Tayyip Erdoğan said: "Why would it be worrying? Every country needs to take certain measures for its own security."
Speaking to reporters at Ankara Esenboğa Airport, Erdoğan said it was "important" for Turkey to choose an entity with which it could be in co-production. He added that negotiations on the missile system had been "largely" completed.
Erdoğan also criticized Dunford's comments during the ruling Justice and Development Party (AK Party) parliamentary group meeting on Tuesday. "Greece has been using S-300 missiles for years, why does it cause concerns when Turkey acquires S-400 systems?" Erdoğan asked, saying that Turkey and Russia have reached an agreement on the issue.
The S-400 system was introduced in 2007 and can carry three types of missiles capable of destroying ground and air targets, including ballistic and cruise missiles. It can track and engage up to 300 targets simultaneously and has an altitude ceiling of 27 kilometers (17 miles).
A fellow NATO member, Greece, deployed Russian S-300 batteries from its southern island of Crete in 2007.
Dunford also said that the U.S. takes into consideration Ankara's concerns of Washington's support for the PKK Syrian affiliate the Democratic Union Party (PYD) and its People's Protection Units (YPG) militia: "We are not only showing effort to clear Raqqa of [Daesh], but also minimizing Turkey's concerns in order to maintain our long-term relations." He added that any political or military solution in Syria will be done by taking Turkey's long-term security concerns into consideration.