"The fast-attack submarine USS Helena is in Guantanamo Bay, Cuba as part of a routine port visit," the US Southern Command said in a social media post, referring to the American naval base on the island.

"The vessel's location and transit were previously planned," it added.

The Russian nuclear-powered sub Kazan -- which Cuba says is not carrying nuclear weapons -- arrived in the Cuban capital Wednesday. It was accompanied by the frigate Admiral Gorshkov, as well as an oil tanker and a salvage tug.

The unusual Russian deployment so close to the United States -- just 90 miles off the Florida coast -- comes amid major tensions over the war in Ukraine, where the Western-backed government is fighting a Russian invasion.

The United States military said it has been monitoring the Russian ships but that they pose no direct threat.

During the Cold War, Cuba was an important client state for the Soviet Union. The deployment of Soviet nuclear missile sites on the island triggered the Cuban Missile Crisis of 1962, when Washington and Moscow came close to war.