There are growing concerns large-scale war games planned next month by Russia with its neighbor Belarus could be a cover for something very sinister by Vladimir Putin — perhaps another Crimea.

There is alarm in Europe that the Russian president could use the military exercises as a sort of Trojan horse or pretext for an annexation of Belarus, a former Soviet republic. Putin has had an increasingly acrimonious relationship with Belarusian President Alexander Lukashenko, particularly since Russia annexed the Ukrainian territory of Crimea.

"Russia is billing it as modest exercises under 13,000 troops, but everything points to probably the largest military exercise in post-Soviet history," said Leon Aron, resident scholar and the director of Russian studies at the American Enterprise Institute, a Washington think tank.

According to Aron, these types of exercises preceded Russia's invasion and later conflict with Georgia, a former Soviet republic that before the war was getting closer to Washington. Similarly, Russia used military exercises as a cover for its assault on the former Soviet republic of Ukraine.

Russia insists its quadrennial Zapad (or Russian for west) joint military drills scheduled Sept. 14-20 will include 12,700 troops and are designed to "test military coordination." However, the New York Times reported last month the entire exercise could involve up to 100,000 people when also including "security personnel and civilian officials."

"We urge Russia to share information regarding its exercises and operations in NATO's vicinity to clearly convey its intentions and minimize any misunderstandings," Pentagon spokesman Johnny Michael told CNBC.

Playing out in the background, though, are concerns from Estonia and its other Baltic NATO neighbors that the Russian 2017 Zapad military exercises have a hidden agenda.

Indeed, Vice President Mike Pence during a recent visit to Estonian capital of Tallinn said: "Russia seeks to redraw international borders by force, undermine democracies of sovereign nations and divide the free nations of Europe."

In April, Reuters quoted then-Estonian Defense Minister Margus Tsahkna as saying his country and other members of NATO obtained intelligence that Russia planned to send troops and resources to Belarus and that when they leave, they will not remove all the equipment and leave some permanent forces behind.

"For Russian troops going to Belarus, it is a one-way ticket," Tsahkna told Reuters. "This is not my personal opinion, we are analyzing very deeply how Russia is preparing for the Zapad exercises."

Tsahkna also was quoted as saying Moscow asked for about 4,000 rail cars to Belarus to transport tanks and other military hardware for the war games. German reports have indicated that is 1,000 more rail cars than the 2013 Zapad.

"Unfortunately, the Russians have a big habit of actually doing operational activities under the guise of war games," said James Carafano, vice president of foreign and defense policy studies at the Heritage Foundation, the Washington think tank. "This goes back to the days of the Soviet Union. So it definitely makes people nervous."

Carafano, who advised the Trump transition team on foreign policy, said the upcoming Zapad military maneuvers also are drawing attention because Russia is getting out from having to invite outside observer nations by claiming it will have less than 13,000 soldiers in the drills. Also, by holding several smaller drills at once Moscow skirts the international treaty known as the Vienna Document and could potentially have the 100,000 people.

"We defer to Russia obviously for anything specific to their military exercises and posture," the Pentagon official said. He also indicated Russia has "conducted several large-scale snap exercises along NATO's eastern flank with little to no notice and in a non-transparent manner."

Then again, Russian media claim Baltic states of Lithuania and Latvia — two former Soviet Republics now part of NATO — are sending observers to the upcoming drills. CNBC reached out to NATO for comment.