The headline and first paragraph of this article were updated on December 1, 2016 to reflect that the exercises took place some distance from Norway's border with Russia, outside the town of Bardufoss.

Kirkenes, Norway (CNN) An American tank full of US Marines crashes through the silence of the sub-zero pine forest around Bardufoss, far above the Arctic Circle, as unidentified drones hover overhead and yellow and green smoke fills the freezing air.

The troops' target? A bunker up ahead, manned by Norwegian soldiers. Shots ring out as the Marines advance, crunching through the snow beneath gray winter skies.

It's all role-play, of course -- the maneuvers are part of a training exercise, but one jarringly imbued with the new reality along NATO's northernmost border with Russia.

Some 300 US Marines are due to be based in Norway on a rotational basis from January, for a year, as part of a package of measures intended to reassure one of NATO's most easterly members.

Although they'll be about 1,000 kilometers (621 miles) from the border, the plan is for them to bolster the readiness of new "pre-positioned" tanks and weaponry stored throughout the year in underground caves.

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Collective defense

But before that deployment, the US and Norway are conducting exercises above the Arctic Circle, taking Abrams tanks further north than they've ever been before, as part of more than a week of joint exercises.

And they are being watched intensely: Norwegian police are investigating more than 10 sightings of unidentified drones spotted observing the US and Norwegian maneuver.

A Norwegian army spokesman, Ole Johan Skogmo, said they were investigating whether the drones were related "to interested locals or other nations."

And the reason for all this activity? In a word: Ukraine.

"In 2014, that was a clear sign that Russia has stepped in to an area where they are willing and able to use military power," says Brigadier Eldar Bernil, of the Norwegian Army. "Suddenly we have changed focus in particular from what was going on in Afghanistan to collective national defense."

US Marines advance towards their target -- a bunker manned by Norwegian allies.

A senior Norwegian security official told CNN there are increasing concerns about the threat posed by Russia, after their actions in Crimea and eastern Ukraine, where small scale separatist rebellions were backed up with the discreet and deniable use of Russian military might.

"We are talking about hybrid warfare," said the unnamed official, "which is warfare under the threshold of war, where you challenge the nature of democracy, where you have free access to social media.

"It's a war without Article 5," he said, referring to collective defense, the central tenet of NATO, which states that an attack on one member is an attack on all NATO allies.

"Suddenly when the tension rises, you bring in the soldiers, like you did in Crimea, and say, 'From now on, I am responsible and I will take care of you as long as you do what I say.'"

US troops take Abrams tanks further north than they've ever been before.

Crimea scenario

Skiing along the wire fence that marks part of the frontier, in the fading winter light, Norwegian border guards are on the lookout for their Russian neighbors.

They do spot troops occasionally. "It happens. You just salute them," says one guard. Would they like to talk to the soldiers? "Probably, but it's illegal."

There are two strategies intended to deter a Crimea scenario in Norway:

Firstly, the presence of armored vehicles, which would enable them to confront the little green men effectively, and would force their backers, Russia, to introduce armored vehicles into the conflict, making it clear that any attacking forces have state backing.

Secondly, the presence of US Marines, ensuring that any interference in Norwegian territory automatically involves the United States in the fight.

And this heightened threat comes at a time of unprecedented political uncertainty in Washington.

Yellow and green smoke floats through the Norwegian forest during the war games.

Break with tradition?

US President-elect Donald Trump's suggestion that he may re-evaluate the nature of the NATO alliance -- which he has labeled "obsolete" -- has frayed nerves here in Norway.

At a remote border post on a snowy peak, within sight of Russia, a Norwegian soldier surveys the scene below through binoculars. He's heard all about Trump's election, he tells us, but he's not allowed to talk about it -- or what it might mean.

Under Article 5 any member in need is guaranteed military support from the US, but Trump has argued that the agreement "is costing us a fortune" and has hinted that he may consider pulling out, potentially leaving allies on Europe's eastern border unprotected in the face of an attack.

"A year ago -- [we would have] no doubt" about America's commitment to Article 5 and our defense, the Norwegian official told CNN. "[The] US is and will be in the future our most important ally."

Yet he went on to add that -- if Mr Trump's policies match his campaign rhetoric -- "he is about to break a very long tradition in American foreign policy."

"If he is going to do what he has said during his election campaign, he will have to change the US way of dealing with foreign countries and create a new security model."

The US is to deploy 300 Marines to Norway from January 2017.

'Strategic messages'

The official conceded that "Trump is right when he says we have to pay a larger portion" of NATO's funding. Allies are supposed to spend at least 2% of GDP of defense, but only five nations -- the US, Greece, the UK, Estonia and Poland -- meet that target.

But with Russia resurgent, now may be exactly the wrong moment to step away from an alliance that has lasted more than 60 years.

The Norwegian official says that over the past two years, "we have gradually seen more and more ... strategic messages being sent [by Moscow]."

He said that after Russia's intervention in Ukraine in 2014, "we have seen a more aggressive pattern when it comes to [Russian planes] flying down the [Norwegian] coast. Submarine activity has picked up to a degree that we haven't seen since the Cold War."