Kiev-loyal regional police chief Vyacheslav Abroskin said constant artillery bombardments were razing the strategic railway hub of Debaltseve, where Ukrainian forces were clinging 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.
"After Minsk the offensive operation of Russia intensified significantly," Poroshenko said, speaking in English.
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The ceasefire, due to take effect from 2200 GMT today, will be the first test of the commitment by Kiev and pro-Russian separatists to the freshly-inked peace plan.
But 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.
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.
Ukrainian security spokesman Andriy Lysenko said that rebels backed up by regular Russian troops were trying to "achieve important tactical goals to extend their territory" ahead of the ceasefire.
Lysenko said that seven soldiers were killed and 23 wounded in clashes over the past 24 hours, while rebel and government officials said three civilians had died.
Obama has warned he could start arms supplies to Ukraine if the new peace deal collapses.