The flight falls under the Open Skies Treaty, an international accord aimed at promoting military transparency through reciprocal, unarmed observation flights over each of the 34 signatory countries' terrains.

Due to tensions with Russia, this is the first Open Skies flight there since November 2017, Pentagon spokesman Lieutenant Colonel Jamie Davis said.

"Russia is aware of the flight," he said.

"Six Russian Federation observers are on board the US aircraft to monitor all phases of the flight."

The flight, which is due to end Friday, is being conducted with an unarmed OC-135B aircraft.

In December, the US and other nations conducted an "extraordinary" Open Skies flight over Ukraine in a mission to show support for Kiev following Russia's seizure of three Ukrainian naval vessels in the Sea of Azov.

The Open Skies Treaty has 34 national members and gives countries "the right to gather information about the military forces and activities of other" signatory countries, the Pentagon said.