Ferocious fighting rages in Ukraine as Kiev and the US accuse Russia of fuelling a rebel onslaught to grab territory in the hours before a truce began.
Kiev-loyal regional police chief Vyacheslav Abroskin said constant artillery bombardments were razing the strategic railway hub of Debaltseve, where Ukrainian forces were hanging on.
"The rebels are destroying the town of Debaltseve. There are non-stop artillery bombardments of residential areas and buildings. The town is in flames," Abroskin wrote on Facebook.
Ukraine's Azov volunteer battalion also reported fierce clashes just to the east of the vital government-held port city of Mariupol and said that the village of Shyrokyne had been "practically destroyed" by shelling.
The ceasefire, due to take effect from 2200 GMT Saturday (0900 AEDT Sunday), will be the first test of the commitment by Kiev and pro-Russian separatists to the peace plan that was inked on Thursday in Minsk after marathon talks between Ukraine President Petro Poroshenko and Russian leader Vladimir Putin.
With Kiev and Washington claiming Russia was spearheading a separatist push to conquer more territory and government forces digging in, there were fears over whether the truce would be observed at all, despite both sides pledging to honour the truce at the appointed time.
On Saturday, Putin re-affirmed his commitment to the ceasefire in a call with its diplomatic co-sponsors, German Chancellor Angela Merkel and French President Francois Hollande.
According to the French presidency, the three leaders, and Putin all confirmed "the need for the ceasefire scheduled for midnight to be effective".
"Putin said the rebels were ready for the ceasefire," it added.
Donetsk rebel chief Alexander Zakharchenko - one of two leaders of rebel forces the West views as Russian puppets - earlier ordered his fighters respect the truce, but ward off any attacks "with all force and means."
On Saturday, Zakharchenko also warned any attempts by Ukraine troops who have been encircled in Debaltseve to escape the city after the ceasefire takes hold will be viewed as an act of aggression, and violation of the truce.
Zakharchenko justified that position by noting "there is not a word about Debaltseve in the Minsk accords."
The UN Security Council was meanwhile expected to meet on Sunday for an emergency session, diplomats said.
The United States said the Russian military had deployed large amounts of artillery and multiple rocket launcher systems and was using them to shell Ukrainian positions.
The fragile deal is seen as the best hope of ending the conflict, which has killed at least 5480 people and ratcheted East-West tensions to levels not seen since the Cold War.
The new Minsk agreement is broadly similar to an earlier failed deal in September and is fraught with potential pitfalls.
Both sides have to begin withdrawing heavy weapons from the frontline within two days of the start of the ceasefire to establish a buffer zone between 50 and 140 kilometres wide, depending on the range of the weapons.
Under the Minsk agreement, Kiev will also begin retaking control over the approximately 400-kilometre stretch of Russia's border with rebel-held Ukraine, but only after local elections are held.
The border is entirely under Russian and rebel control and is used, according to Kiev, as a conduit for separatist supplies.
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