NATO Secretary General Jens Stoltenberg (File photo: CGTN)

ANKARA, May 6 (Xinhua) -- NATO Secretary General Jens Stoltenberg said on Monday that he is worried about Turkey's plans to deploy the Russian S-400 missile defense system despite of US objection.

"The purchase of any equipment is the decision of each NATO ally," Stoltenberg said in a joint news conference with the Turkish Foreign Minister Mevlut Cavusoglu in the capital Ankara.

"But I am concerned about the potential consequences of the decision to buy S-400. Because the US has made it clear to impose sanctions," he added.

Meanwhile, Stoltenberg welcomed Turkey's key role in NATO's training mission in Iraq.

NATO helps protect Turkey with air and missile defense systems and enhanced patrols by Turkey's Airborne Warning and Control System surveillance planes, he said.

"Over the years, NATO has invested more than 5 billion US dollars in military facilities in Turkey," the secretary general said, adding that this included important infrastructure such as airfields, naval bases, and radar sites.

Recalling that Turkey proposed to establish a joint working group in order to eliminate hesitations of NATO and the US over its purchase of S-400 systems, Cavusoglu said that "the US would accept our proposal if it is sincere with these concerns."

Turkish President Recep Tayyip Erdogan has repeatedly said that Turkey won't step back from the S-400 deal, despite mounting pressure from Washington, including sanctions threats.