NEW : CNN : eight instances of Russian aircraft in European airspace in late Wednesday . Groups of Russian aircraft detected over areas of the Atlantic Ocean , North , Black seas . NATO : `` Unusual level of air activity over European airspace '' NATO official tells NATO the recent flights could be possible prelude to Russian air exercises .
NEW : NATO : More than 19 instances of Russian aircraft in European airspace in 24 hours . Groups of Russian aircraft detected over areas of Baltic , North , Black seas . NATO : `` Unusual level of air activity over European airspace '' NATO official tells CNN the recent flights could be possible prelude to Russian air exercises .