The US military on Wednesday said Russian subs were patrolling off the US East coast but Moscow shrugged off the report, saying its vessels always operate legally.
US Northern Command issued a brief statement that the submarines were being monitored while stopping short of any protests against Russia.
"NORAD (North American Aerospace Defense Command) and US Northern Command are aware of Russian submarine activity off the East coast operating in international waters," said James Graybeal, spokesman for NORAD and Northern Command.
"We have been monitoring them during transit and recognize the right of all nations to exercise freedom of navigation in international waters according to international law," he said in the statement.
The New York Times first reported the presence of two Russian nuclear-powered, Akula class submarines off the American coast, the first such move in several years that carried echoes of Cold War tensions.
Russia neither confirmed or denied that its submarines were patrolling near US territory, but suggested there was undue "hysteria" in this case.
"Activities of Russian submarines in the world's oceans outside their own waters do not violate international maritime law and are within normal practice," a military-diplomatic source told ITAR-TASS news agency.