(Reuters) - NATO aircraft tracked Russian strategic bombers over the Atlantic and Black Sea on Wednesday and sorties of fighters over the Baltic in what the Western alliance called an unusual burst of activity at a tense time in East-West relations.

In all, NATO said in a statement, its jets had intercepted four groups of Russian aircraft in about 24 hours since Tuesday, and some were still on maneuvers late on Wednesday afternoon.

"These sizeable Russian flights represent an unusual level of air activity over European airspace," the alliance said.

A spokesman stressed there had been no violation of NATO airspace -- as there was last week when a Russian spy plane briefly crossed Estonia's border. But such high numbers of sorties in one day were, he said, rare in recent years.

In the biggest exercise, four Tupolev Tu-95 strategic bombers, a Cold War icon since the 1950s equivalent to the U.S. B-52, flew out over the Norwegian Sea in the early hours of Wednesday, accompanied by four refueling tanker aircraft.

Norwegian F-16s were scrambled and tracked the formation, which eventually broke up, with six planes heading back toward Russia and two Tu-95s flying on south over the North Sea, where they were intercepted by British Typhoons. Portuguese F-16s later tracked them in the Atlantic before they turned for home.

A Norwegian military spokesman said: "We see Russian aircraft near our airspace on a regular basis but what was unusual is that it was a large number of aircraft and pushed further south than we normally see."

In a second incident, two Tu-95s accompanied by two fighter jets were being tracked by Turkish aircraft over the Black Sea on Wednesday afternoon, while flights of seven Russian warplanes were monitored on Tuesday and Wednesday over the Baltic Sea.

On Tuesday, German and Danish planes were involved in tracking them as well as aircraft from non-NATO states Sweden and Finland. On Wednesday, Portuguese F-16s posted in the Baltic intercepted a similar group of fighters and fighter-bombers.

NATO said it had conducted more than 100 such intercepts of Russian aircraft this year so far, about three times as many as in 2013, before the confrontation with Moscow over separatist revolts in ex-Soviet Ukraine soured relations.

President Vladimir Putin has committed to reinvigorating Russia's armed forces, which had been undermined by the economic troubles that followed the collapse of the Soviet Union. Tension over Ukraine has seen the U.S.-led NATO alliance step up its vigilance, especially on its eastern frontiers with Russia.

The spokesman said there was no particular reason for concern over Russian warplanes exercising their right to fly in international airspace but that such sorties were shadowed by NATO aircraft as a precaution and to protect civil air traffic.