NATO said Wednesday it had tracked and intercepted four groups of Russian warplanes "conducting significant military manoeuvres" in European airspace over the past two days.
"These sizeable Russian flights represent an unusual level of air activity over European airspace," NATO said.
The planes, which included strategic bombers, fighters and tanker aircraft, were detected over the Baltic Sea, North Sea/Atlantic Ocean and the Black Sea on Tuesday and Wednesday, it said.
NATO allies sent up aircraft to intercept and identify the Russian planes which were continually tracked on the ground as well, it said in a report from its SHAPE military headquarters in Mons, western Belgium.
The report made no mention of the Ukraine crisis nor linked the increased activity to it.
However, Russia's intervention in Ukraine, which NATO has said constitutes the most serious threat to US-Europe security since the Cold War, has sent tensions soaring.
NATO's eastern members, many of them such as Poland and the Baltic states once ruled from Moscow, have been particularly nervous and the US-led alliance has put in place additional aircraft and personnel on a rotating basis to reassure them.
At a summit last month, the 28 NATO leaders agreed the alliance would have to step up its readiness levels in response.
Russia's intervention and the speed with which it was able to bring military pressure to bear on Kiev in support of pro-Moscow rebels in the east surprised many and badly rattled countries on NATO's eastern flank.