NATO said Wednesday that it had intercepted a large number of Russian aircraft flying close to European airspace in the past two days, in an �unusual� series of incidents that brought Russian bombers as far afield as Portugal.

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In at least one of the four incidents, the aircraft had switched off their transponders and had not filed flight plans with civilian air traffic controllers. That means that civilian air traffic control cannot track them, potentially creating a risk for civilian planes.

That incident took place around 3:00 a.m. in Western Europe on Wednesday, when four Tu-95 long-range strategic nuclear bombers and four Il-78 tanker aircraft flew over the Norwegian Sea. Norwegian F-16 fighter jets scrambled to intercept them. Six of the planes returned to Russia, but two of the bombers skirted the Norwegian coast, flew past Britain � sending Typhoon fighter jets to scramble in response � and then finally looped west of Spain and Portugal, attracting Portuguese F-16s. Then the two bombers appeared to return to Russia.

NATO detected and monitored four groups of Russian military aircraft conducting significant military maneuvers in European airspace over the Baltic Sea, North Sea/Atlantic Ocean, and Black Sea on 28 and 29 October 2014. These sizable Russian flights represent an unusual level of air activity over European airspace.

(Reuters) - Portugal scrambled F-16 jet fighters for the second time this week on Friday to intercept Russian bombers in the international air space along its coast in a new sign of an unusual burst of Russian activity next to NATO's southern borders.

Defence Minister Jose Pedro Aguiar-Branco told reporters the Portuguese jets successfully intercepted, identified and accompanied two Russian aircraft out of the international air space under the Portuguese jurisdiction.