The U.S. commander responsible for North America and the Navy’s top officer warned lawmakers this month about Russia’s increasingly capable and active submarines, which they say are operating closer to U.S. shores.

Air Force Gen. Glen VanHerck and Navy Adm. Michael Gilday are the latest officials to express concern about Russia’s submarine fleet, which is smaller than its Soviet predecessor but has improved considerably in recent years.

Asked about threats “below the nuclear threshold” at a June 15 House Armed Services subcommittee hearing, VanHerck — who leads U.S. Northern Command and NORAD — said Russia and China are “developing capabilities” below that threshold “to hold the homeland at risk.”

“Those capabilities would include very quiet submarines,” VanHerck said. “Russia just fielded their second Sev-class submarine, which is on par with ours.”