Russia and its key ally Belarus are to start major joint military drills on Friday, putting NATO on edge days after Poland accused Moscow of escalating tensions by firing attack drones through its airspace.

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.

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