American Boeing OC-135B on mission (Photo: US Air Force)

The Open Skies Treaty, signed in 1992 and implemented in 2002, allowed 35 member states, including Russia and the US, to conduct surveillance flights in each other's airspace to openly collect information on each other's forces and military activities. Both Moscow and Washington regularly accused each other of violating the agreement.

In May, 2020, the US started the process of withdrawing from the Treaty and this process was completed in November last year.