Russia-led Collective Security Treaty Organization (CSTO) member-states attended military drills in parts of Belarus on Friday.

The CSTO exercises included 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 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.