Poland will shut its border with Belarus at midnight on Thursday in response to a major Russian military exercise, Prime Minister Donald Tusk said Tuesday.

Moscow holds the Zapad exercise, a joint drill with Kremlin-allied Belarus, roughly every four years to simulate a clash with NATO countries in Eastern Europe.

Tusk said the “very aggressive” drills had prompted Warsaw’s decision to shut the border with Belarus.

“We are facing a growing number of provocations from Russia and Belarus,” he added.

The war games, running Sept. 12–16, will involve tens of thousands of Russian and Belarusian troops in maneuvers close to Poland and Lithuania.

Some drills — near the Belarusian village of Gozha and in the Russian exclave Kaliningrad’s Dobrovolsk — will take place just a few dozen kilometers away from the NATO countries’ borders.

Russian and Belarusian troops will also practice planning to use Oreshnik nuclear-capable missile systems.

Tusk said a key “target” of the drills is the Suwałki Gap, the NATO-controlled, 70-kilometer choke point separating Belarus and Kaliningrad that Russia would need to take over in the event of an all-out war.

It is not the first time Poland has clamped down on its border with Belarus. Warsaw has shut border checkpoints in recent years amid mounting tensions with Minsk and last year suspended access to asylum for migrants crossing into the country via Belarus, accusing the country of orchestrating a crisis on the Polish border as a form of hybrid warfare.