NATO says it has detected “unusual” activity in European airspace in the past two days.

Speaking in Brussels Wednesday, a NATO military spokesman said that four groups of Russian warplanes, including Tu-95 strategic bombers and MiG-31 jets, have been “conducting significant military maneuvers” over the Baltic Sea, North Sea and the Black Sea.

He said Norwegian, British, Portuguese, German and Turkish fighters were sent up to intercept and identify the Russian planes, adding that such measures “are standard procedure when an unknown aircraft approaches NATO airspace.”

Tensions between NATO and Russia have escalated since Russia annexed the Crimean Peninsula from Ukraine in March.

There have been more than 100 intercepts of Russian aircraft in 2014 alone, three times more than in 2013.

NATO’s eastern members, including Poland and the Baltic states, have been particularly annoyed by Russian military activity.  In response to their concerns, the U.S.-led alliance has increased the number of aircraft and personnel to monitor the airspace on a rotational basis.

  airspace, european, incursions, nato, Russia