TALLINN, Estonia — More than 2,000 troops from a Russia-led security alliance opened military exercises Friday in parts of Belarus near the borders of NATO countries.

The exercises of the Collective Security Treaty Organization include troops from Russia, Belarus, Kyrgyzstan, Kazakhstan and Tajikistan. Armenia is also a member of the CSTO but did not send any of its forces; in January, Armenia said it would not host CSTO drills this year amid tense relations with Russia.

Belarus’ defense ministry said the exercises, which are to last through Wednesday, are to prepare for joint operations, including responses to a nuclear accident.

The exercises are taking place in three western Belarus regions that border NATO members Poland and Lithuania. Polish Foreign Minister Zbigniew Rau said the purpose of the military exercises was to destabilize the region.

Belarus late Friday claimed that a Polish military helicopter flew at low altitude about 1200 meters (0.75 miles) into Belarusian airspace in the Grodno region, one of the areas where the exercises are taking place. Polish charge d’affaires Wojciech Filimonowicz was summoned for a complaint, Belarus said.

Belarus hosts Russian forces and equipment that have been used in the war in Ukraine, although Belarusian forces have not taken part. Russia also says it has deployed tactical nuclear weapons in Belarus.

Ukraine’s Joint Forces commander Gen. Serhii Naiev said Ukraine has strengthened its border with Belarus for the exercises.