If US allies have been unnerved by President Donald Trump's comments on the 69-year-old military alliance, massive manoeuvres at the end of October are intended as a show of force and unity in the face of an increasingly assertive Russia.

The Trident Juncture 18 exercise will draw in 45,000 troops, Secretary General Jens Stoltenberg said -- the biggest movement of NATO personnel and vehicles since the 1991 collapse of the Soviet Union.

Speaking shortly before the announcement at NATO headquarters in Brussels, Mattis told reporters in Paris that the US was "keenly aware of the dangers close to your home".

"Every NATO ally is awake today to the reality of Russia's malicious behaviour. Every ally has now increased defence spending," he said.

Eastern European countries have grown increasingly worried since the Russian annexation of Crimea in 2014 that Moscow is eyeing their territory.

NATO allies have beefed up their presence in the region over the past four years, operating rotating garrisons in eastern Europe and the Baltic states.

The forthcoming military exercises are set to simulate the defence of a NATO member state from a "fictional" adversary, according to Stoltenberg.

But the troops, tanks, ships and planes are headed for Norway, the North Atlantic and the Baltic -- opposite Russia.

While huge in scale, the Juncture 18 operation will be far smaller than the Vostok-18 exercise staged by Russia and China last month, which involved 300,000 troops and 1,000 aircraft.

NATO's effort will involve troops from Britain, North America and continental Europe, equipped with 150 aircraft, 70 vessels and around 10,000 land vehicles.

- 'Actions speak loudest' -

In a possible veiled reference to Trump's repeated verbal attacks on NATO, Mattis told reporters that when it comes to America's relationship with the alliance, "actions speak loudest".

"The US has acted, showing that our commitment to the transatlantic alliance remains iron-clad," he told a press conference alongside his French counterpart Florence Parly.

He noted that in defence spending plans for 2019, US lawmakers have not cut military spending in Europe "by a single cent, instead maintaining once again the highest levels of commitment since the 1989 fall of the Berlin Wall".

Washington has maintained the number of US troops permanently stationed in Europe -- more than 60,000 -- "while adding additional capability", he said.

Mattis' words of reassurance come a day ahead of a gathering of NATO defence ministers in Brussels.

Trump has repeatedly taken European allies to task for not spending enough on their militaries, particularly Germany.

Before taking office, he called NATO "obsolete" and soon after a tumultuous summit last July he questioned whether the US would honour the alliance's founding principle of mutual defence for newest member Montenegro.

Parly told reporters that "Europe is not part of the problem, Europe is part of the solution".

"The United States is making sharing the burden a priority. It is also a French priority," she said.

"It's a question of making the whole Atlantic alliance function better."

Despite tensions between Trump and US allies in Europe, NATO foreign ministers are set to mark the 70th birthday of the North Atlantic Treaty Organisation in Washington in April.