The exercises, codenamed “Zapad”, or “West”, are believed by Nato to have started on Thursday and are due to last until September 20.
They are being conducted on military ranges in Belarus, western Russia, Russia’s exclave of Kaliningrad and in the Baltic Sea.
The Kremlin claims only 12,700 troops are involved in the deadly drills, along with up to 70 aircraft and helicopters, up to 680 units of military hardware including 250 tanks, up to 200 artillery pieces, multiple rocket launchers and mortars, and up to 10 warships.
But Nato says the war games are significantly larger than Moscow has publicised – potentially involving some 100,000 troops.
The exercises also involve firing nuclear-capable ballistic missiles, NATO claims, and officials say the drills will simulate a conflict with the US-led alliance intended to show Russia’s ability to mass large numbers of troops at very short notice in the event of a conflict.
And while Nato says it is monitoring the biggest war games since 2013 with “calm and confidence”, many are unnerved about what they see as Moscow testing its ability to wage war against the West.
But Russia’s defence ministry insists the exercises are of a purely defensive nature and are not a threat to any third country or group of countries.
Kremlin spokesman Dmitry Peskov said: ”We reject complaints of these exercises not being transparent. We believe that whipping up hysteria around these exercises is a provocation.”
“It is a normal practice for any country to hold such exercises. Everything is being held in line with international law.”
Russian President Vladimir Putin may also visit one of the stages of these drills, he said.
The drills come as US officials plan a Pentagon review of America’s ageing nuclear arsenal, which could require a trillion dollar modernisation plan.
US military chiefs have also noted that America’s nuclear modernization is lagging far behind Russia’s upgrade of its own nuclear triad.
General Paul Selva, vice chairman of the Joint Chiefs of Staff, told Congress in August he believed Moscow was already two-thirds through its nuclear modernization process.
The West is already fearful of the potential break out of World War 3 following the recent nuclear tests from crazed dictator Kim Jong-Un in North Korea.
The tyrannical regime recently tested its sixth nuclear weapon, a H-bomb believed to be five times more powerful than the device dropped on Hiroshima.
And in retalitaion, the US had threatened “fire and fury” – with Generla James Mattiss warning total annihilation was a possibility.