Those drills followed the massive "Sea Breeze" war games in the same area.
The upcoming Russian exercise has also caused considerable concern in Ukraine, where President Petro Poroshenko warned that Zapad-17 could be utilized to further threaten the territorial integrity of his country. Kiev has stepped up security on Ukraine's borders.
During the drills, the joint Russian-Belarus operations are set to target a theoretical adversary attempting to undermine the government in Minsk and establish a separatist stronghold in western Belarus.
North Atlantic Treaty Organisation 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. Russia denies that, but admits that some Russian "volunteers" are helping the rebels.
Russian Federation has claimed that the exercise, which is being conducted in cooperation with the army of Belarus, will include no more than 13,000 troops.
The 29-nation Western alliance also has an air-policing mission over the Baltic republics.
Moscow has insisted that the long-planned drills, which it is calling Zapad 2017, are defensive and relatively small, involving only around 12,000 troops. But some Western analysts say the number is more like 100,000 when you consider the other personnel involved.
Approximately 1,650 service members from 15 different countries are participating in Exercise Rapid Trident 2017 which began Monday and will last until September 23, Pentagon spokesman Johnny Michael told CNN.
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If, after Zapad-17, Moscow decides to force the issue by enlarging its military infrastructure in Belarus nonetheless, it would be the clearest indication since Putin's 2014 seizure of Crimea that he has chose to transform Belarus into a military outpost against the West.
Russia's defence ministry confirmed that the exercises were under way.
Tensions between Moscow and the West have spiraled since Russia's 2014 annexation of the Crimean peninsula, and Ukraine is among the nations anxious Zapad 2017 could lead to further landgrabs.
Washington sees the operation as a demonstration of U.S. commitment to Europe's security amid so-called Russian aggression in the region. His counterpart, Belarus' President Alexander Lukashenko is an ally of Russia, Zwack said, "but a prickly one". "Polish Defence Minister Antoni Macierewicz has termed the exercise as a threat to Poland".
"We reject complaints of these exercises not being transparent", Kremlin spokesman Dmitry Peskov told a conference call with reporters.
An Estonian MP, Hannes Hanso, said he had spoken to "Belarusian colleagues" about the strategic importance of the "Suwalki Gap" - a border strip separating Kaliningrad from Belarus.
"He's been playing this constant balancing act between the West and Russia", NPR's Kim says. The Belarusian officials, he said, assured him that "their territory would not be used for this kind of thing".