"Russia is not 10 feet tall but they do have assets that keep me vigilant, concerned. One of them is in the undersea domain," Foggo, the commander of US Naval Forces Europe, told reporters at the Pentagon.

While Foggo said Russia's surface fleet, including its aging aircraft carrier, posed little threat -- saying Moscow did "not have a robust capital ship capability" -- he did express concerns about Russian advancements in its development of submarines and cruise missiles.

"We've seen creation of new classes of all sorts of submarines and ships. I'm more concerned with submarine warfare," Foggo said earlier on Friday while addressing the Atlantic Council in Washington.

"Russians have produced the new Dolgorukiy-class submarine. They've produced the Severodvinsk-class submarine. They've produced the new Kilo hybrid-class submarines," Foggo told reporters at the Pentagon.