Russian President Vladimir Putin and his ally Belarusian leader Alexander Lukashenko have extended military drills which were due to end on Sunday.
Belarus cited the “deterioration of the situation” in east Ukraine as one reason for keeping an estimated 30,000 Russian troops on its territory.
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. It has not yet confirmed the Belarus defence ministry statement.
Explosions were heard 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.
A top EU official asked if Moscow was any longer interested in diplomacy.
“The big question remains: does the Kremlin want dialogue?” European Council President Charles Michel asked at the Munich Security Conference.
“We cannot forever offer an olive branch while Russia conducts missile tests and continues to amass troops.”
Mr Putin spoke to his French counterpart, Emmanuel Macron, by phone again on Sunday, after which the French leader called Ukrainian President Volodymyr Zelensky.
According to a US estimate, Russia has mustered up to 190,000 troops including separatist forces in the Donetsk and Luhansk regions.
Mr Putin has been demanding guarantees that Nato will not admit Ukraine, a former Soviet state with close ties to Russia, while the Western alliance denies it poses any threat to Russia.
There are fears that a Russian military intervention could start a war even bloodier than the conflict in eastern Ukraine which has cost at least 14,000 lives.
UK Prime Minister Boris Johnson told BBC News he had seen evidence to suggest Russia was planning “the biggest war in Europe since 1945”.
As explosions boomed out in Donetsk on Sunday morning, separatists 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. It was unclear on what legal basis the Russian investigation was being conducted but Russia has given citizenship to at least 720,000 people in rebel territories.
The separatists and government forces accused each other of violating the ceasefire dozens of time on Sunday. On Saturday, international monitors reported that ceasefire violations had increased dramatically this week.
Tens of thousands of civilians are being evacuated from the separatist territories into Russia while men of fighting age are being mobilised to fight.
Source: Angola Press News Agency