The incident, which took place during the early hours of last Thursday morning, was only made public earlier this week. The Tupolev T1-95 bombers were escorted by two F-16s.
The incident caused additional alarm among national authorities as the bombers do not emit any signals nor do they respond to radio transmissions, which they said could pose serious risks to civil aviation.
The air force added that the bombers did not enter Portugal’s sovereign air space as they flew down to Sagres from northern Portugal and back again.
Portuguese authorities said the Russian planes showed no hostility during the period they were accompanied by the F-16 fighter jets.
This incident comes after Russia’s only aircraft carrier Admiral Kuznetsov and its task force sailed past Portugal late last month as it headed south from the Norwegian Sea on its way to Syria.
The carrier was flanked by several Russian warships and a number of submarines as it sailed past the Portuguese coastline.
Nato recently accused Russia of provocative military manoeuvres, especially in the Baltic region as Russian warplanes stepped up patrols near Nato countries’ airspace.
Portugal reacted to the incident through Defence Minister José Azaredo Lopes, who said nothing pointed towards the situation being of any “particular seriousness.”
Minister Lopes said he thought and hoped it was nothing more than a showing off of the fleet, which passed within Portugal’s exclusive economic zone.
He added “unforeseen situations” trigger “speculation” and questions that should be followed with “maximum attention.”