Air carrier pilot reported a loss of GPS signal for approximately 2 hours while in cruise over Turkey and Iraq.

Narrative

During a flight of a Commercial Fixed Wing from ZZZZ to OKBK at 39000 ft.; Approximately 200 miles from ARTAT in the vicinity of LROP (Bucharest) with a clearance of direct to ARTAT parallel with the P975 airway ATC fault Engine Indicating and Crew Alerting System message illuminated. Non-normal checklists we're completed. It was discovered that both GPS signals were inoperative. ATC was notified. Problem continued intermittently for approximately 15 minutes until the GPS signal was continuously lost and was absent from the vicinity of the Black Sea; across Turkey and part of Iraq; approximately 2 hours; INS/IRUS operated normally and position was independently verified via radial and distance; especially since the flight planned route was within approximately 10 miles of the Iranian border at the closest point. Normal function returned between SOBIL and VAXEN on the UM688 airway. Cause unknown; whether of military interference with GPS or intermittent dual equipment failure. Navigation route and position verified with alternate means. Log book/discrepancy entry made at destination. Monitor and issue warnings to crews if this is a persistent problem in this geographical region.