Moscow and Minsk say the Zapad (West) 2017 exercise, scheduled from September 14 to 20 in Belarus and parts of western Russia, is officially set to involve 12,700 troops, RFE/RL reported.
But Western officials have said the maneuvers could include some 100,000 personnel in what they call a Russian show of power amid the ongoing standoff with the West over Russian aggression in Ukraine.
"This is designed to provoke us, it's designed to test our defenses, and that's why we have to be strong," British Defense Secretary Michael Fallon told the BBC on September 10.
Read alsoNATO's Stoltenberg calls for transparency of Zapad 2017 drillsRussia, meanwhile, has pushed back against what it portrays as Western alarmism over the drills, the first to be held in close proximity to NATO member states since Russia annexed Crimea in 2014.
Moscow insists that the size of the exercise will not cross the 13,000-troop threshold that, under Organization for Security and Cooperation in Europe (OSCE) rules known as the Vienna Document, would require it to notify other countries and open the maneuvers to observers.
Colonel General Andrei Kartapolov, commander of Russia's Western Military District, said in an interview published by the Russian military's official Krasnaya Zvezda newspaper on September 13 that the number of troops and hardware used in the drills "will fully comply with the Vienna Document."
The Zapad exercise is held every four years in rotation with drills in other parts of Russia.
Western governments have responded to Russia's 2014 seizure of Ukraine's Crimean Peninsula and backing of separatists in eastern Ukraine with several waves of economic and other sanctions targeting Moscow.
Read alsoUK defense chief says Zapad 2017 drills aimed at provoking NATONATO has also bolstered its presence in its easternmost member states that were dominated by Moscow during the Cold War and remain concerned about the Kremlin's intentions in the region.
Belarus, where part of the Zapad 2017 exercise is being held, borders Ukraine as well as NATO members Poland, Lithuania, and Latvia. The drills are also being staged in Russia's western enclave of Kaliningrad, which lies between Poland and Lithuania on the Baltic Sea.
NATO Secretary-General Jens Stoltenberg said in Estonia last week that the military alliance would send three observers.
"But these invitations fall short from the transparency required by the OSCE: briefings on the exercise scenario and progress, opportunities to talk to individual soldiers and overflights of the exercise," Stoltenberg told reporters on September 6 during his visit to a NATO contingent in Tapa, Estonia.
"We will monitor the [Zapad 2017] activity closely, and we are vigilant but also calm, because we don't see any imminent threat against any NATO ally," Stoltenberg added.
Russia, which has repeatedly accused NATO of stoking regional tensions through enlargement after the fall of the Iron Curtain and deployments in Eastern Europe in the aftermath of the Ukraine crisis, has called Western concerns about the Zapad drills baseless, saying the exercise is "purely defensive."
Read alsoUkraine boosts security on borders ahead of Zapad-2017 military drills - PresidentKartapolov told Krasnaya Zvezda that in addition to the stated 12,700 troops – around 7,200 from Russia and 5,500 from Belarus – Zapad 2017 included about 70 aircraft and up to 680 pieces of military hardware, including tanks, artillery units, and ships.
During the drills, the joint Russian-Belarus operations are targeting a theoretical adversary attempting to undermine the government in Minsk and establish a separatist stronghold in western Belarus.
This scenario echoes Russian concerns over what Moscow calls Western-orchestrated political revolutions in its backyard, most notably in Georgia in 2003 and in Ukraine, where President Viktor Yanukovych, a Kremlin ally, was ousted in early 2014.
The United States and the European Union have repeatedly rejected such allegations, calling those events the result of grassroots anger against corrupt regimes in the former Soviet republics.