The Western military alliance has said some 90,000 troops will take part in the months-long Steadfast Defender 24 exercise designed to test its defences in the face of Russia's war on Ukraine.

"The alliance will demonstrate its ability to reinforce the Euro-Atlantic area via trans-Atlantic movement of forces from North America," said General Christopher Cavoli, NATO's Supreme Allied Commander Europe.

"Steadfast Defender 2024 will be a clear demonstration of our unity, strength, and determination to protect each other, our values and the rules-based international order."

The exercise is designed to simulate the 31-nation alliance's response to an attack from a rival like Russia.

It will be composed of a series of smaller individual drills and will span from North America to NATO's eastern flank, close to the Russian border.

Some 50 naval vessels, 80 aircraft and over 1,100 combat vehicles will take part.

The exercise -- the biggest since the 1988 Reforger drill during the Cold War -- comes as NATO has overhauled its defences since Russia launched its invasion of Ukraine in 2022.

The alliance has dispatched thousands of troops to its eastern flank and drawn up its most extensive plans since the collapse of the Soviet Union to protect itself from a Russian attack.