(CNN) -

NATO has scrambled fighter jets more than 400 times this year to intercept Russian military flights close to alliance members' airspace in Europe, the alliance's secretary general said this week.

That's a 50 percent increase in Russian air activity over last year and the kind of activity that harkens back to the days of the Cold War, NATO Secretary General Jens Stoltenberg said during a visit to NATO member Estonia on Thursday.

"This pattern is risky and unjustified. So NATO remains vigilant. We are here. And we are ready to defend all allies against any threat," he said at Amari Airbase in Estonia, where U.S., German and Estonian troops were gathered.

Stoltenberg said few of the Russian flights had actually violated the airspace of NATO nations, but he said the way the Russian planes operate threatens civilian aviation in the region.

"They are not filing their air flight plans. They are not turning on the transponders. And they are not communicating with the civilian air traffic control," he said.

"We are calling on Russia to conduct their military air activities in a responsible way and respecting international norms for this kind of air activity," Stoltenberg said.

In a report earlier this month, the European Leadership Network listed more than 40 "close military encounters between Russia and the West" that took place in the eight months from March to October of this year.

Three of those, including a near collision between a Russian military plane and a Swedish passenger aircraft carrying 132 people, were classified as "high-risk" incidents that could have led to direct military confrontation between Russia and the West, according to the report, titled "Dangerous Brinksmanship."

Russia also has said it will expand its military flights, with Defense Minister Sergey Shoigu saying this month that Russian military aircraft would be flying along U.S. coasts and even into the Gulf of Mexico.

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NATO jets are scrambling more for Russians