Esper told a congressional hearing Russia had been blocking the United States from conducting flights over the Baltic Sea city of Kaliningrad and near Georgia that are permitted by the 18-year-old agreement.

"We've also been denied access to military exercise overflights," he said. "I have a lot of concerns about the treaty as it stands now."

The deal, which has 32 other signatories, permits one country's military to conduct a certain number of surveillance flights over another each year on short notice.

The aircraft can survey the territory below, collecting information and pictures of military installations and activities.

The idea is that the more rival militaries know about each other, the lower the chances of conflict.

Esper said he raised the issue last month in the meeting with NATO defense ministers.

"This is important to many of our NATO allies, that they have the means to conduct the overflights," he said.

The Pentagon operates two old OC-135 aircraft for the overflight operations, and Esper acknowledged the need to update the aircraft, even if the Pentagon has not set aside the cash.

"At this point time, until we make a final decision on the path forward, I'm not prepared to recapitalize aircraft," he said.