Russian strategic nuclear bombers conducted at least 16 incursions into northwestern U.S. air defense identification zones over the past 10 days, an unusually sharp increase in aerial penetrations, according to U.S. defense officials.

The numerous flight encounters by Tu-95 Russian Bear H bombers prompted the scrambling of U.S. jet fighters on several occasions, and come amid heightened U.S.-Russia tensions over Ukraine.

Also, during one bomber incursion near Alaska, a Russian intelligence-gathering jet was detected along with the bombers.

“Over the past week, NORAD has visually identified Russian aircraft operating in and around the U.S. air defense identification zones,” said Maj. Beth Smith, spokeswoman for U.S. Northern Command and the North American Aerospace Defense Command (NORAD).

Smith called the Russian flights “a spike in activity” but sought to play down the threat, stating the flights were assessed as routine training missions and exercises.

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