Russia and its key ally Belarus have begun joint military exercises amid tensions with NATO countries after Poland accused Moscow of firing attack drones over its airspace.

The Zapad exercises are taking place at a time when Russian forces are advancing along the stretched front line in Ukraine and escalating air strikes against Ukrainian cities.

NATO's eastern flank members bordering Belarus — Poland, Lithuania, and Latvia — are on high alert because of the exercises, which Belarus says will take place near Borisov, a town east of the capital Minsk.

All three countries have stepped up security measures ahead of the exercises, with Poland ordering a complete closure of its border with Belarus for the duration of the exercises, AFP reported.

"Joint strategic maneuvers by the Russian and Belarusian armies have begun," the Russian Defense Ministry said.

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

He said Poland was closer to "open conflict" than at any time since World War II after Poland and its NATO allies sent fighter jets to shoot down Russian drones flying in its airspace on Wednesday morning.

Moscow downplayed the concerns.

"These are planned exercises that are not directed against anyone," Kremlin spokesman Dmitry Peskov said, rejecting Poland's claim that the exercises were an "aggressive" show of force.

But Ukrainian President Volodymyr Zelensky also issued a warning about Moscow's intentions.

"The significance of such actions by Russia is definitely not defensive and is directed precisely against not only Ukraine," he said in Kiev.

Russia said it had intercepted 221 Ukrainian drones over its territory during the night. | BGNES