Russia and Belarus began major war games Thursday, an operation involving thousands of troops, tanks and aircraft on NATO's eastern edge practicing how to hunt down and destroy armed spies, among other maneuvers.

The Zapad (West) 2017 maneuvers, which are mainly taking place in Belarus this year, have caused concern among members of the Western military alliance and in neighboring countries. Some NATO members, including the Baltic states and Poland, have criticized a lack of transparency about the exercises and questioned Moscow's real intentions.

Russia and Belarus say the exercises, which run until Sept. 20, involve 5,500 Russian and 7,200 Belarusian troops. Russian military officials have said up to 70 aircraft and about 250 tanks, 200 artillery systems and 10 navy ships will also be involved.

Estonian Defense Minister Juri Liuk, however, says Moscow could deploy up to 100,000 troops.

"Leaving weapons in Belarus means the Russian army could prepare bases for a sudden broad attack ... right at the NATO border," Lithuanian officer Darius Antanaitis said.

While the Baltic nations fear the Zapad maneuvers may lead to a surprise Russian attack, the exercises have also been criticized by Belarusian opposition leaders. They say Russia could use the occasion to position a large, permanent contingent of troops in Belarus, leaving the country at the mercy of any armed confrontation involving Moscow.

The exercises began Thursday night with units simulating hunting down and destroying reconnaissance agents belonging to illegal armed groups, according to Oleg Belokonev, the Belarusian Deputy Defense Minister.

"Command points have been set up and fully-functioning command systems created," Belokonev told journalists at a press conference in Minsk, the capital.

Valery Gerasimov, the chief of the General Staff of the Russian armed forces, formally notified NATO of the beginning of the exercises on Thursday evening, according to Russian media. NATO Secretary-General Jens Stoltenberg told NATO troops in Estonia last week that the alliance will be closely monitoring Zapad exercises.

Russia-West relations nosedived to their lowest level since the Cold War in recent years after Moscow's 2014 annexation of Ukraine's Crimean Peninsula and its support of separatists fighting the Ukrainian government in eastern Ukraine, clashes that have left over 10,000 people dead.

Russia's Defense Ministry said Thursday that elite parachute units in several Russian cities had been placed on alert to be deployed during the exercises.

Organizers have invented three "aggressor countries" — Veishnoriya, Lubeniya and Vesbasriya — to whose attacks the Russian and Belarusian militaries will simulate a response. The Baltic States and Poland fear that these monikers are just poorly disguised terms for their own countries.

Poland's National Security Bureau head, Pawel Soloch, said Thursday the exercises were a demonstration "of the Russian state's capacity to hold full-scale war action."

"The degree of mobilization is really impressive," Soloch said on private Radio Zet.

Lithuanian President Dalia Grybauskaite, who often criticizes Russian leaders, said the war games are a sign the Kremlin is preparing for conflict with NATO.

"We are anxious about this drill ... it is an open preparation for war with the West," Grybauskaite told reporters.

There is also unease in Kiev, and Ukraine is currently conducting its own military exercises. Ukrainian President Petro Poroshenko has said that Zapad 2017 appears to be a "preparation for an offensive war on a continental scale."

Both Moscow and Minsk have said repeatedly that the exercises are not a danger for neighboring countries.

"We are not threatening anyone," Oleg Voinov, an adviser to the Belarusian Defense Minister, told journalists Thursday. "We have chosen military bases that are significantly removed from the borders with Ukraine, Poland, Lithuania and Latvia."

Dmitry Peskov, a spokesman for Russian President Vladimir Putin, said Thursday that Russia had been completely open and transparent about its military's involvement in the exercises.

The most recent Zapad exercises, which occur every few years, took place in 2013, just before Russia's 2014 annexation of Crimea. Russia had leased a naval base in Crimea from Ukraine prior to its seizure, and used troops deployed there to quickly take over the Black Sea peninsula.

Some people think fears of Russian aggression are being blown out of proportion.

"Worries over Zapad are overkill. Russians will not seek confrontation, because they know that NATO will be watching this event closely and is certainly ready to react," said Kestutis Girnius, a Vilnius University political analyst.