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.
France said the pair agreed to work for a ceasefire on the contact line in eastern Ukraine.
There were more explosions in the east Ukrainian conflict zone through the night and into Sunday.
Detonations could be heard from the separatist-held city of Donetsk while both sides said they had come under heavy shell fire.
As explosions boomed out in Donetsk on Sunday morning, Russian-backed rebels in Luhansk accused government forces of crossing the front line to mount an attack which killed two civilians. No proof was given for the allegation but Russian investigators opened an inquiry.
The rebels and government forces accused each other of violating the ceasefire dozens of time on Sunday. Two Ukrainian soldiers were killed on Saturday as international monitors reported that ceasefire violations had increased dramatically this week.
Thousands of civilians are being evacuated from the separatist territories into Russia while men of fighting age are being mobilised to fight.
Speaking as she prepared to leave Donetsk for Russia by bus with her four-year-old daughter, an evacuee who gave her name as Tatyana told Reuters news agency: « It’s really scary. I’ve taken everything I could carry. »
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