"I have a lot of concerns about the treaty as it stands now", Esper told the US Senate Armed Services Committee on Wednesday. "At this point of time, until we make a final decision on the path forward, I am not prepared to recapitalize aircraft. I want to make sure we understand what the direction is".
Esper accused Russia of denying US monitors access to the Kaliningrad region, territory adjacent to Lithuania and Poland, and Georgia and military exercise areas.
The Defence Department chief said he had raised the issue of alleged Russian non-compliance with the Open Skies Treaty at the NATO defence ministerial last month and urged the allies to speak out more clearly about that.
The Open Skies Treaty was signed in Helsinki on 24 March 1992 and currently comprises 35 member states. They may conduct surveillance flights over the territories of the other parties to the treaty in order to gather information about potential and actual military activities.
Washington has recently announced that it is considering withdrawing from the agreement, claiming a potential breach of the agreement by Russia and security concerns. Moscow has denied all the allegations.