NATO interceptions of Russian aircraft in European skies have tripled this year / Photo from Wikimedia

Speaking at a press conference in Brussels on Tuesday, Stoltenberg said he would not draw conclusions about the reasons from Russia’s intensified military activity in European airspace and in Ukraine, but said that the alliance was responding properly.

"I will not speculate about the intentions of Russia and its goals, but we are reacting properly, and I believe it is important for us to continue applying security measures," Stoltenberg said.

He also confirmed that Russia has increased activity "not only in the air but also on the border with Ukraine."

"I think we'll see a demonstration of strength, especially increased activity of aircraft. We’re doing what we have to do," Stoltenberg said.