"The Bombardier Artemis II was spotted for the second time this week," flying west to east over the southern Black Sea, the source told TASS.
Last Tuesday, it spent nine hours on a similar route, matching its summer flight endurance "record."
It was earlier spotted near the Kaliningrad Region, circling over Lithuania, Poland and the Baltic Sea.
It is now flying at around 9,000 meters, outside civil aviation corridors, and has entered no national airspace except Romania, where it took off. The aircraft is based in Constanta, the source added.