The "Zapad" exercises also come as Russian forces are grinding across the sprawling front line in Ukraine and escalating aerial attacks on Ukrainian cities.

NATO's eastern flank members that border Belarus -- Poland, Lithuania and Latvia -- are on high alert over the drills, which Belarus says will be held near Borisov, a town east of the capital Minsk.

All three countries have ramped up security ahead of the exercises, with Poland ordering the complete closure of its border with Belarus for their duration.

Polish Prime Minister Donald Tusk warned of "critical days" for his country.

He said Poland was closer to "open conflict" than at any point since World War II, after Poland and its NATO allies scrambled jets to down Russian drones flying across its airspace early Wednesday.

Moscow has downplayed the concerns.

"These are planned exercises, they are not aimed against anyone," Kremlin spokesman Dmitry Peskov told reporters Thursday, rejecting Poland's claim that the drills were an "aggressive" show of force.

But Ukraine's President Volodymyr Zelensky also issued a warning over Moscow's intentions.

"The meaning of such actions by Russia is definitely not defensive and is directed precisely against not only Ukraine," he said in Kyiv on Thursday.

- Just a show? -

Usually held every four years, the 2025 iteration of Zapad is the first during the conflict in Ukraine, and is to run until September 16.

Moscow sent around 200,000 troops to similar drills in 2021, just months before it launched its Ukraine offensive.

But this year's Zapad is expected to be much smaller, since hundreds of thousands of Russian troops are deployed in Ukraine.

Belarus had said in January that 13,000 troops would be involved in the drills, but in May it said the number was to be reduced by around half.

According to Tusk, the exercises are designed to simulate the occupation of the Suwalki corridor, a geographical gap that stretches along the border between Poland and Lithuania, flanked by Belarus and the Russian exclave of Kaliningrad.

The corridor is considered a vulnerability for NATO and could potentially be the first target of any Russian attack.

Belarusian President Alexander Lukashenko has dismissed that fear as "utter nonsense".

Earlier this year, state media quoted Belarus's defence minister as saying the drills had been moved away from the borders with Poland and Ukraine to "reduce tensions".

But Poland has nonetheless shut its few remaining border crossings with Belarus -- drawing criticism from Moscow -- and restricted air traffic along its eastern border.

Lithuania and Latvia have also announced partial airspace closures.

Russia's stationing of tactical nuclear weapons in Belarus has given the drills a new dimension.

Minsk said in August the exercises would involve Russia's new experimental nuclear-capable missile, dubbed Oreshnik, as well as nuclear strike training.

Moscow-based military analyst Alexander Khramchikhin told AFP the importance of the drills was being overblown, calling them "just a show" with little "special significance".

He said that similar exercises were held at this time every year, rotating between different parts of Russia and previously including nuclear simulations.

But Vassily Kashin, a military analyst and member of the Kremlin-linked Russian International Affairs Council, said the exercises were "both a demonstration and real combat training".

"We must be ready to defend Belarus, if necessary," he told AFP, noting that Poland and its allies planned to hold their own counter-drills through September.

Kashin added that the practice of rival drills by Russia and NATO's eastern members at the same time was probably here to stay, "just as it was during the Cold War".