(NEW YORK) -- A number of Russian aircraft were intercepted by NATO on Friday, the second time this week that Russian aircraft prompted the scrambling of NATO fighter jets.
A U.S. defense official told ABC News that NATO tracked "multiple sets" of Russian aircraft, including fighter jets, bombers and refueling tankers. At least two TU-95 bombers were among the Russian planes tracked, after being spotted west of the United Kingdom. The planes had traveled as far south as Portugal before turning back.
The official said that the Russian planes did not violate airspace, but were tracked because they filed no flight plans.
Such incidents "pose the potential risk of escalation," Pentagon spokesman Rear Adm. John Kirby said Friday, adding that the U.S. and NATO are watching the Russian flights "very, very closely."
NATO says that there have been approximately 100 intercepts involving Russian aircraft in 2014 -- about three times the amount in all of 2013.
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