A Russian Tu-95 Bear H flies over International waters off the coast of Norway. Four Russian strategic bombers triggered U.S. air defense systems near Alaska this week, with two aircraft coming within 50 miles of the California coast. (Associated Press)

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.