Washington, Aug 6 (DPA) Two Russian submarines patrolling off the Atlantic coast of the United States do not pose a threat, the Pentagon said Wednesday.

The US was monitoring the attack subs more than 300 km off the US eastern seaboard, where they are operating legally in international waters, Pentagon spokesman Geoff Morrell said.

'The largest question is, is it of concern to us? And the answer to that is no,' Morrell said, acknowledging that such patrols have been rare since the end of the Cold War.

'We watch it. We're mindful of it. But it doesn't necessitate anything more than that,' Morrell said, adding the US military has ways to 'derive where they were going'.

The Russian Navy has begun to flex its muscle in the Western Hemisphere in the last year, making port calls to Cuba and Venezuela while irking Washington.

Attack submarines are designed to fight at sea and do not carry ballistic nuclear missiles.

The New York Times reported that US defence and intelligence sources considered the manoeuvres as showing a surprising level of assertiveness by the Russian military.