Some 50,000 soldiers, 10,000 vehicles, 65 ships and 250 aircraft from 31 countries were taking part in Trident Juncture 18, which is aimed at training the Atlantic Alliance to defend a member state after an aggression.

"In recent years, Europe's security environment has significantly deteriorated," NATO Secretary General Jens Stoltenberg said.

"Trident Juncture sends a clear message to our nations and to any potential adversary. NATO does not seek confrontation but we stand ready to defend all allies against any threat," he told reporters on Wednesday.

While the "potential adversary" has not been officially identified, Russia was on everybody's minds.

The country, which shares a 198-kilometre (123-mile) border with Norway in the Far North, has repeatedly flaunted its military strength in recent years.

The Russian army has annexed Crimea, helped destabilise eastern Ukraine, beefed up its military capabilities in the Arctic, and conducted its biggest ever exercises in the Far East in September.

- 'Anti-Russian' exercise -

The Russian embassy in Oslo said it considered Trident Juncture an "anti-Russian" exercise.

"Such activity... comes across as provocative, even if you try to justify it as being of a purely defensive nature," it said.

For months, Moscow has been annoyed by the growing Western military presence in the region. The United States and Britain have been increasing their deployments in Norway to accustom their troops to cold weather combat.

Russian foreign ministry spokeswoman Maria Zakharova, speaking in early October, condemned what she termed as NATO's "sabre-rattling", and vowed Moscow would take "retaliatory measures".

"The main NATO countries are increasing their military presence in the region, near Russia's borders," she said.

"Such irresponsible actions are bound to lead to a destabilisation of the political situation in the North, to heighten tensions," she said.

Tensions already flared on Saturday after US President Donald Trump announced he was abandoning a Cold War-era nuclear treaty, a move which Russia warned could cripple global security.

Accusing Russia of developing a new missile, the SSC-8, Trump threatened to increase the US nuclear weapons arsenal.

- Article 5 secured? -

Despite concerns about Trump's commitment to the Alliance -- particularly regarding the "Article 5" mutual defence clause -- the US military is contributing the biggest contingent to Trident Juncture, with more than 14,000 troops, including an aircraft carrier.

"We exercise in Norway but of course the lessons ... from Trident Juncture are also relevant for other countries," Stoltenberg said.

In addition to NATO's 29 member countries, Norway's neighbours Sweden and Finland will join the exercises, which run through November 7.

"We do not see any military threat against Northern Europe today but we live in uncertain and unpredictable times," wrote five Nordic defence or foreign ministers in a joint column published in several newspapers Thursday.

"A more confident Russia has demonstrated both its willingness and ability to use military might to achieve its own strategic goals," they wrote, praising the importance of security cooperation among their countries.

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

Stoltenberg said he hoped Russia would "avoid dangerous behaviour".