Russian President Vladimir Putin and his ally Belarusian leader Alexander Lukashenko have extended military drills which were due to end on Sunday.
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.
The BBC's Eastern Europe correspondent Sarah Rainsford said the announcement - made by the Belarus defence ministry - is another strong signal that Russia is not prepared to back down in its stand-off with Western countries over Ukraine.
In a phone call on Sunday, Russian leader Vladimir Putin told French President Emmanuel Macron the Russian troops would eventually leave Belarus, the French presidency said. Mr Putin blamed the Ukrainian military for the escalation of tensions in the Donbas.