He added that the United States was modernizing its nuclear arsenal, investing in new technologies such as drones and a new long-range bomber, as well as lasers and new systems for electronic warfare.
The defense chief hinted at additional new weapons that would be "surprising ones I really can't describe here."
Additionally, "we're updating and advancing our operational plans for deterrence and defense given Russia's changed behavior," Carter said.
Russia annexed Crimea in 2014 and has been supporting a pro-Moscow separatist conflict in eastern Ukraine.
Russian jets also started bombing Syria on Sept. 30.