Russia is bolstering its underwater battle capabilities in a direct challenge to US forces in the Atlantic and Mediterranean, a top US admiral warned Friday.

Admiral James Foggo, who heads NATO's Allied Joint Force Command in Naples, said Russia is investing heavily in its submarine fleet and wants to build an "asymmetric" threat to the US and NATO.

While the Russian navy knows it cannot compete on the level of aircraft carriers and larger surface vessels, "they have continued to do research and development and recapitalize the undersea domain," Foggo told Pentagon reporters.

"They see it as one (in) which there is a challenge, and that challenge is the United States Navy and the United States submarine force."

Foggo said the Russians currently have six Kilo-class diesel-electric attack submarines in the Black Sea and Eastern Mediterranean.

When asked if the Navy knows where all Russian subs are at any given time, Foggo, who used to command an attack submarine, declined to elaborate.

"I can tell you that we hold an acoustic advantage and we will continue to do that," he said, referring to the listening technologies that allow sailors to locate subs.