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A Russian spy ship from the Cold War era has been spotted "loitering" off the coast of the US just 30 miles from a major submarine base.

US officials were keeping a close watch on the SSV-175 Viktor Leonov intelligence-gathering vessel as it travelled through international waters.

The spy ship is armed with surface-to-air missiles and equipped with technology that can intercept communications and collect data on US Navy sonar capability.

On Wednesday morning the ship was 30 miles south of Groton, Connecticut, home of the Naval Submarine Base New London.

(Image: Getty Images South America) (Image: Fox News)

Grotton is known as the "home of the submarine force" and is the primary east coast submarine base for the US.

A US official told Fox News that the ship's position also put it just south of Montauk on the eastern tip of New York's Long Island.

The official said the Viktor Leonov was "loitering" in international waters.

On Tuesday the ship was spotted just 70 miles from the coast of Delaware.

Its position on Wednesday was the furthest north it has ever travelled up the US eastern seaboard, and it was last seen off the US east coast about two years ago, Fox News reported.

That time it was lurking near another US submarine base in Kings Bay, Georgia, while in January 2015 it was spotted in Havana, Cuba.

Officials at the Pentagon believe the ship will sail south along the US coast and return to the Caribbean, Fox News reported.

The Viktor Leonov is based on the Barents Sea in the Arctic but in recent weeks reportedly had crossed the Atlantic Ocean and stopped in Jamaica as part of a journey that had been planned for months.

US Senator Chris Murphy, a Democrat from Connecticut, expressed concern over the sightings.

He wrote on Twitter: "Russia is acting like it has a permission slip to expand influence, test limits of reach. Questions are obvious: does it, and if so, why?"

There have been a number of provocations involving Russia in recent years.

Last Friday four Russian jets flew past a US Navy destroyer, the USS Porter, in the Black Sea, ignoring radio calls from the ship.

US-Russia relations have been under intense scrutiny following President Donald Trump's victory over Democratic challenger Hillary Clinton last November.

Earlier this week Trump's national security adviser, Michael Flynn, resigned after just three weeks on the job as it emerged that he discussed US sanctions on Russia with Moscow's ambassador to the United States before the president took office and later misled Vice President Mike Pence about the conversations.

Flynn was seen in Moscow as a leading advocate of warmer ties with Russia.

A member of Trump's administration said the president knew for weeks that Flynn had misled the White House about his contacts with Russia but did not immediately force him out, Reuters reported.

Trump has suggested there is a media conspiracy and denied the notion of a "Russian connection" in response to a New York Times report which said members of his presidential campaign had contact with Russian intelligence officials.

Trump tweeted on Wednesday: "This Russian connection non-sense is merely an attempt to cover-up the many mistakes made in Hillary Clinton's losing campaign."

The Kremlin has also denied the reports.