"They inform us that they are out there," said Adm. James G. Foggo III, commander of the US Navy in Europe, for the increased presence of Russian submarines in the North Atlantic and Arctic Oceans.
"They work in a much larger number and in places that have not worked before."
It will be NATO's largest exercise for decades, which will include 50,000 soldiers, 10,000 vehicles, 250 aircraft and 65 vessels, including an American carrier operating north of the Arctic Circle for the first time nearly 30 years.
Tensions between Russia and the West are at a high level since the Cold War amid the poisoning of former Russian secret agent Sergey Sprill in England, allegations of Russian intervention in the 2016 elections and western sanctions in Moscow following the annexation of the Crimea.
But Foggo, who oversees Trident Juncture, said the exercise was not a threat to Russia, noting that NATO and Russia troops would be away more than 700 kilometers during the maneuver. NATO, he added, had invited observers from Russia and Belarus to attend the exercise.
"I want to be there because this brings the strength of the alliance," said Foggo.
As exercise plays, it will include air, ground and naval operations, including anti-submarine warfare.
Russia is not yet equal
Fogo said he believes Russia has over 40 submarines, more than 20 concentrated in its northern fleet capable of operating in the North Atlantic and Arctic.
To watch Russian submarines, NATO airplanes make a flight every other day from a renewed US base at Keflavik International Airport.
Iceland's Foreign Minister Thór Thórdarson said in a speech in Stockholm in January that alliance aircraft are operating outside the country with increased frequency, starting with Keflavik for a total of 153 days in 2017, a steady annual increase of just 21 days in 2014.
Established in 1951, the US Navy Terminal in Iceland was deactivated in 2006 as NATO shifted its center to Europe south to the Mediterranean. However, the threat caused by the turbulent Russia and its underwater fleet worried the US military commanders and brought the Americans back to this island nation, located between Greenland and the United Kingdom.
To be found from the bases of the Russian Arctic in the open Atlantic, Moscow submarines have to pass from Iceland.
Fogo says these submarines are a major headache for NATO leaders.
"The Russians continued to invest in research and development and produce very capable submarines, it was our most capable rival," said the American admiral, who spoke with CNN in an exclusive interview.
Russia says its fleet is defensive and indispensable to ensure the country's security.
At this year's "Submarine Day" in March, Vice-President Oleg Burtsev, the former head of Russian naval forces, spoke about the importance of strengthening the country's underground fleet.
"This is because the plans of our country and our army leadership are to ensure that we are able to reliably resist any possible enemy from all directions," Burshev said, according to the Russian news agency Tass.
And another former senior naval commander said that Russia has some work to do to fit the submarine fleet that NATO allies can gather.
"I think the quality of our fleet is high enough now, but its amount is still not enough," Tass Adm said. Vladimir Komoyeftov, former head of the Black Sea Fleet of Russia.
A large part of NATO's problem with the Russian sub-fleet is its own, said Carl Schuster, former naval captain and university professor at the University of Hawaii.
"A large part of the current threat (the Russian slave fleet) is based on expanding its operations and operating areas at a time when NATO countries have reduced fleets and fleet operations," Schuster said, calling him "a serious threat only because it was ignored until recently to focus on other security issues. "
A new generation of threats
Foggo says the new generation of submarines in Russia is extremely capable and dangerous. Among the younger ones are the Borei class: virtually silent nuclear vessels capable of launching ballistic missiles. The Borei class is a major pillar of Russia's underwater nuclear deterrent force, similar to the Ohio US missile ballistic missile.
"This is beyond any doubt the future of our Navy Strategic Nuclear Forces team," said Admiral Chief of Naval Forces in Russia recently. Vladimir Korolev, at the baptism of a new submarine Borei.
But Russia is also in the process of modernizing many of its oldest submarines, such as diesel-electric Kilo fuel. They can now stay below the water longer and are capable of carrying four cruise missiles, which were successfully shot at ISIS targets in Syria, according to the Russian army.
"They carry the Kalibr cruise missile, a very capable weapon system, and from any of the parts operated by the Russians, can target any capital in Europe," said Foggo.
"Will they do that? I do not think so, but we still have to be informed where it is at all times," he said.
Surprise said the concern gives Russia an edge.
"Moscow's aggressive actions and intentions will determine the time and place of a crisis, while Western nations must be present and ready to respond at any time," he said.
That is why NATO has methodically intensified operations in Iceland.
Chess in the ocean
The US spends $ 34 million on upgrading facilities in Keflavik, which will allow the Navy to develop more often the P-8 Poseidon Surveillance and submarine aircraft.
But even with twin-engine jets that perform regular surveillance in the North Atlantic, finding Russian submarines is not an easy task.
"The ocean is big … It's a chess game between the subordinate and all the assets trying to find him," said Cmdr. Rick Dorsey, the tactical coordinator for one of the US P-8 units operating from Iceland, told CNN. "It's a combination of many tasks, from many different units."
"We work with ships, we work with other aircraft, we work with other nations to help the image," Dorsey said.
It is the kind of teamwork among allies that Adm. Foggo wants to encourage, applauding the UK and Norway to acquire their own P-8 aircraft and inviting NATO members to invest in research and development to maintain a competitive edge over Russia. "We must continue to challenge them wherever they are and to know where they are," he said.
"We can no longer assume that we can sail unpunished to all the oceans."