Michael Carpenter, deputy assistant secretary of defense for Russia, Ukraine, and Eurasia, affirmed a February analysis by the RAND Corporation, which determined that it would take Russian troops a maximum of 60 hours to reach the capital cities of Estonia or Latvia.

"I don't know that we're significantly more advanced now than when the RAND report came out," Carpenter said during a Senate Foreign Relations Committee hearing. "But I'm confident by the end of 2017, when we have an additional armored brigade combat team worth of force posture on the eastern flank of the alliance, that we will be."

The report concluded that NATO "as presently postured … cannot successfully defend the territory of its most exposed members," and urged the deployment of roughly seven brigades to the Baltics.

Carpenter said that the alliance was working on boosting its defense posture in Eastern Europe, but that Russia possessed a geographical advantage.

"Russia clearly possesses a time-distance advantage, if it were to decide to be an aggressor in the Baltic states," he said. "We are making the investments … to have forces prepositioned, along with more fighting equipment, so that we are better able to deter Russian aggression in the first place."

Poland and other NATO states opened a large-scale, 10-day military exercise Monday, the most extensive in eastern Europe since the cold war. Simultaneously, Russia has been beefing up its armed forces near Ukraine, and has warned against any increased NATO presence on its borders.

In July, NATO members will attend a summit in Warsaw where they will "take decisions to strengthen [NATO's] deterrence and defence and step up efforts to project stability beyond its borders," according to a press release.