America is emphasising its increasing military presence in Europe as Nato's biggest military exercise since the Cold War continues in Norway.
Trident Juncture involves all 29 members plus Finland and Sweden.
The US has recently increased its military personnel based in Europe.
As part of the manouevres, a US aircraft carrier is sailing above the Arctic Circle for the first time in decades; the USS Harry S. Truman is leading a US strike carrier group.
General Curtis Scaparrotti, NATO's Supreme Allied Commander in Europe said:
"Just look what United States is doing. This carrier strike group is part of the exercise. But even in the last couple of years, we have increased the number of personnel we have in Europe. We've just announced another 1,500 increase. The money that is called the European Defence Initiative is investmed here in Europe to ensure that we have ready troops that are working close to NATO allies for the defence of the Euro-Atlantic.
Russian military analyst Alexander Goltz says Russia is watching with concern this response to recent Russo-Chinese exercises: "I think that the Russian authorities did not count on such a response from NATO when they plotted all the actions in Crimea and Ukraine. Within 700 kilometres of Russian territory, NATO conducts large-scale military exercises of the type last seen in the Cold War. Here we see for the first time well prepared brigades, not just battalion tactical groups, brigades deployed for military operations. It's serious."