Trident Juncture 18, which runs until November 7, is aimed at training the Alliance to mobilise quickly to defend an ally under attack.

The head of NATO's Allied Joint Force Command, US Navy Admiral James Foggo, said the exercise was intended to "show NATO is capable to defend against any adversary. Not a particular country, anyone."

Russia, which carried out its biggest ever military exercises in September in the Far East, has not been officially identified as the intended adversary, but it is on everyone's minds after the 2014 Ukraine crisis.

"Russia doesn't represent a direct military threat to Norway," Norwegian Defence Minister Frank Bakke-Jensen told AFP.

"But in a security situation as complicated as we have today... an incident elsewhere could very well heighten tensions in the North and we want to prepare the Alliance in order to avoid any unfortunate incidents," he added.

The exercises come after President Donald Trump has repeatedly complained that other NATO members do not contribute enough money to the 69-year-old alliance, although Defense Secretary Jim Mattis reassured allies of America's "iron-clad" commitment earlier this month.

- 'Sabre-rattling' -

While the exercises will take place at a respectful distance from Norway's 198-kilometre (123-mile) border with Russia in the Arctic, Moscow has expressed anger over the manoeuvres.

Russia was already touchy over the fact that -- independently of Trident Juncture 18 -- the United States and Britain have been increasing their troop presence in the Scandinavian country to acclimatise them to combat in the chilly Arctic.

And tensions between Moscow and Washington have flared in recent days after Trump announced he was abandoning a Cold War-era nuclear treaty, a move which Russia warned could cripple global security.

When at full strength, 700 US Marines will be on rotation on Norwegian soil.

"The main NATO countries are increasing their military presence in the region, near Russia's borders," Russian foreign ministry spokeswoman Maria Zakharova said, slamming "the sabre-rattling".

"Such irresponsible actions are bound to lead to a destabilisation of the political situation in the North, to heighten tensions," she said, vowing Moscow would "take the necessary retaliatory measures to ensure its security."

Under President Vladimir Putin, the Russian army has already beefed up considerably in the Arctic.

Military air bases have been built or refurbished, and new radar and anti-aircraft missile systems have been installed.

In addition, the backbone of the Russian navy, the Northern Fleet, is due to receive five new warships, five support vessels, and 15 aircraft by the end of the year, according to Defence Minister Sergei Shoigu.

- Military 'choreography' -

"Russia's military strength has pretty much returned to what it was during the Cold War," Francois Heibsbourg of France's Foundation for Strategic Research (FRS) told AFP.

"In a way, NATO is also in the process of returning to what it was."

"It's a pretty mechanical engagement," a "return to a kind of choreography", he said. But Trident Juncture 18 is "in no way destabilising," he added.

The exercises, involving NATO's 29 members plus Sweden and Finland, are nonetheless imposing, with substantial means deployed.

The 50,000 troops will be backed by 10,000 vehicles, 250 aircraft and 60 ships, including a US aircraft carrier.

"The core exercise area is 1,000 kilometres (600 miles) from the Russian border, and air operations could take place up to 500 kilometres away from the border," Norwegian Lieutenant General Rune Jakobsen said.

"There should not be any reason for the Russians to get scared or see this as anything other than a defensive exercise."

Two Russian and two Belarus military observers have been invited to watch the manoeuvres.

The British contingent hit the road for five days to travel to the exercises.

"It demonstrates ... to our NATO allies that we're prepared to move across Europe when needed and to show that we have the capability to do so," Major Stuart Lavery told AFPTV.