A statement cited the "deterioration of the situation" in east Ukraine as one reason for keeping an estimated 30,000 Russian troops in Belarus.
The move will add to fears that Russia plans an invasion of Ukraine, which shares a long border with Belarus.
Western leaders have accused Moscow of seeking a pretext to send in troops.
Russia has denied it plans to invade its neighbour.
US Secretary of State Antony Blinken said the extension of military exercises shows the world is on the brink of war.