RAF jets were scrambled over Scotland after a quick reaction alert on Tuesday.
Typhoons from RAF Lossiemouth were sent from their base near Moray, using the 1307 squawk code, which signals NATO air policing.
The jets were seen crossing northern Scotland, and heading towards the Outer Hebrides.
SkyScanWorld, an aircraft and satellite tracker, reported that the scramble was possibly a practice operation involving a Dassault Falcon with tanker support from an RAF Airbus Voyager.
Quick reaction alerts are incidents that have occurred since the Cold War-era, and usually involve RAF crews shadowing military Russian aircraft flying near UK airspace.
RAF Lossiemouth has been contacted for comment.
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