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.
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.
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.”
Separatist leaders accuse the government of planning an offensive to regain control of the territories – an allegation rejected earlier by the head of the Organization for Security and Co-operation in Europe (OSCE) security watchdog.
- EXPLAINER: What is a false flag attack?
- CONTEXT: Satellite images show Russian activity near Ukraine
- BACKGROUND: Is Russia going to invade Ukraine?
“We deplore the spreading of disinformation about an imminent military action by Ukrainian government forces; this critically affects the civilian population in the conflict zone,” Helga Schmid, the OSCE’s secretary general, said in a joint statement with Polish Foreign Minister Zbigniew Rau.
Mr Zelensky has said his nation will not respond to provocations. Two Ukrainian soldiers were killed in the conflict zone on Saturday.
He accused Western leaders of a “policy of appeasement” towards Moscow and demanded security guarantees for Ukraine, which officially aspires to join both Nato and the EU.
The big guns are back
By Orla Guerin, BBC News, Novotroitske
The frontlines in eastern Ukraine are rarely silent – at the very least there is sniper fire between Kremlin-backed rebels and Ukrainian forces. But now the big guns are back. In recent days there has been a barrage of artillery and mortar fire. We heard it ourselves at frontline positions in Novotroitske.
We reached the trenches on foot, crossing flat ground where snow has given way to sucking layers of mud. Two fresh-faced troops led the way, standing stock still at the rumble of shelling a few kilometres away.
At a sandbagged lookout position 24-year-old Taras told us there had been three days of heavy incoming fire. “I think Putin is doing it on purpose to provoke us,” he said. “The situation has gotten worse, but we tolerate it, and we wait.”
Ukrainian forces say they have orders not to return fire – except in exceptional cases – as Russia is “looking for any excuse to invade”.
Taras had a deadpan expression, and a phlegmatic approach to a possible invasion. “They might raze us to the ground,” he told me, “and they might not. And we don’t know if we will have support or not. If it gets too hard, we will open fire too.”