NATO Secretary-General Jens Stoltenberg said that Turkey's plan to buy Russia's S-400 air defense missile system reflects the country's "national decision."

In an interview with Anadolu Agency (AA), Stoltenberg said, "It is a national decision by each and every ally, including Turkey, what kind of military capability they would like to invest in."

"This is a national Turkish decision. What is important for NATO is the interoperability; meaning that different systems can work together. There has been no request [to] integrate the S-400 into a joint NATO system. It goes without saying that integrating such a system would be difficult," he added.

Last December, Turkey announced it had concluded an agreement with Russia for the purchase of two S-400 systems by early 2020. The system will cost Turkey $2.5 billion.

The S-400 system has been in the inventory of the Russian military since 2007. It is Russia's most advanced, long-range anti-aircraft missile system and can carry three types of missiles, capable of destroying several types of targets, including ballistic and cruise missiles apart from aircraft.

It is not the only air defense system that Turkey is considering. Last July, Turkey signed an agreement with fellow NATO members France and Italy to develop its own national missile defense systems.

"There are other NATO systems, which I know Turkey is considering, and I welcome that," the NATO chief said.

About NATO-Turkey relations, especially since the launch of Operation Olive Branch in Syria's Afrin region, Stoltenberg said Turkey continues to be a "highly-valued NATO ally." "Turkey is contributing to our collective security, to our missions and operations in many different ways. I thank Turkey for that," he said, and termed the country's security concerns as "legitimate."

"NATO provides support to Turkey. We have missions, military presences in Turkey. We have our surveillance AWACS planes, flying over and helping Turkey. We also have the air-defense batteries called SAMP-T, provided by Spain and Italy," Stoltenberg added.

"We are grateful to Turkey for their support to NATO. We also show solidarity with Turkey by NATO's military presence in the country, which has been increased over the last years to reflect the instability and threats Turkey is facing from south, from Daesh, from terrorism, especially in Iraq and Syria," he further added.