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General Curtis Scaparrotti says the United States and its NATO allies are ready to respond to a growing threat from Russia.

The top American commander in Europe was speaking as a U.S. aircraft carrier sails to the Norwegian Sea for the first time since 1987, Sky News reported.

The USS Harry S Truman – powering through the North Sea following a visit to Portsmouth last week – is preparing to join NATO's largest military exercise since the end of the Cold War.

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The ship's presence is part of a renewed focus by the alliance on the waters between Europe and the U.S. following a rise in Russian submarine and warship activity.

Asked what message the 100,000-ton carrier was sending to Moscow, General Scaparrotti said: "It is a demonstration of our capabilities… We are ready and we are trained and we will protect the Atlantic."

The Supreme Allied Commander Europe was speaking as he toured the ship with Jens Stoltenberg, the secretary general of NATO, and other top officials.

"Russia has set out to increase their presence and also they are modernizing their force and that is not something I would think would be unexpected but it is something we do need to pay attention to," General Scaparrotti said.

"We need to continue to increase our capabilities and to train so that we remain dominant in war-fighting capabilities. Strength is how we deter conflict and our job is actually to prevent and we will do that with a strong alliance."

Armed with 49 fast jets and escorted by a further nine U.S. warships, the USS Truman will play a part in Exercise Trident Juncture, which begins on October 25 in and around Norway and runs until November 7.

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Some 50,000 military personnel from all 29 members of the NATO alliance, as well as from Finland and Sweden, will be involved in the war game.

They are expected to simulate a scenario in which a hostile country – Russia is at the forefront of commanders' minds – challenges a member of the alliance, triggering an Article 5 response.

Article 5 is the founding principle of NATO's collective defense, meaning that an attack on one ally is an attack on all.

Mr Stoltenberg, also speaking aboard USS Truman, said it was important for allies to be prepared.

"To keep our nations safe in an unpredictable world we need to keep our alliance strong," he said, speaking to a small group of journalists.

"We do need to have the training and we need to train together in all domains – at sea, in air on land and in cyber space."

NATO has been regrowing its ability to exercise large numbers of soldiers, sailors and airmen to counter the threat of war with a peer opponent – a requirement that was allowed to decline following the collapse of the Soviet Union.

The rethink was triggered by President Vladimir Putin's annexation of Crimea in 2014 and the ongoing conflict in eastern Ukraine.

It has been given greater urgency as evidence mounts of unconventional attacks by Russia – using cyber, disinformation on social media and other measures – to meddle in Europe and the United States as part of an effort to grow influence and divide the West.

Military strength alone is not enough to deter hostile activities.

Britain earlier this month led an unprecedented move to expose the actions of Russia's military intelligence agency – the GRU.

This included a botched attempted by four GRU officers to hack into the computers of the international chemical weapons watchdog.

It followed the attempted assassination of former Russian double agent Sergei Skripal and his daughter in Salisbury in March.

The British government has held Russia responsible. The Kremlin denies any involvement.