Stoltenberg said the exercise would simulate the defence of a member state from a "fictional" adversary, but the troops, tanks, ships and planes are headed for Norway, the North Atlantic and the Baltic -- opposite Russia.

It will be the biggest such movement of NATO personnel and vehicles since at least the 1991 collapse of the Soviet Union, although still smaller than the Vostok-18 exercise staged by Russia and China last month.

"The exercise is defensive, and it is transparent," the NATO leader told reporters on the first day of a two-day meeting of the 29-member alliance's defence ministers at its new Brussels headquarters.

"All members of the Organisation for Security and Cooperation in Europe, including Russia, have been invited to send observers," he said.

The operation will bring alliance troops -- equipped with 150 aircraft, 70 vessels and around 10,000 land vehicles -- from Britain, North America and continental Europe up through northern Europe and Scandinavia to NATO's northeastern flank at the end of the month.

The Western allies have stepped up their military posture, with rotating garrisons in eastern Europe and the Baltic States, in the four years since Russia annexed the Ukrainian region of Crimea.

Exercises like Trident Juncture are designed to practice moving a larger force forward quickly in the event of any outside intervention against a NATO member.