A police chief said the strategic town of Debaltseve was suffering a ferocious bombardment by pro-Russian rebels.
Renewed fighting is also reported near the southern port city of Mariupol.
Meanwhile, the US ambassador to Ukraine tweeted recent satellite images that he said showed Russian artillery near Debaltseve.
Geoffrey Pyatt also said on Twitter that Russian units along the border were preparing a large shipment of supplies to separatist fighters.
He said that the rebels were now better armed than some Nato countries.
Ukraine and the West accuse Russia of sending troops and weapons to help the separatists in Ukraine's eastern Donetsk and Luhansk regions - a claim the Kremlin vehemently denies.
Petro Poroshenko warned the deal to end the war in the east was in "great danger"
"The militants are destroying Debaltseve," Donetsk region
police chief Vyacheslav Abroskin was quoted as saying on the censort.net
website."The shelling of residential areas and civilian buildings continue. The city is on fire. There has been a direct hit from Grads (rockets) on the city police station."
In Mariupol, reports said pro-Russian rebels were using artillery and tanks to attack nearby villages.
The port city lies between rebel-held eastern areas and the southern Crimea peninsula, which Russia annexed last March.
Ukrainian Deputy Defence Minister Petro Mekhed said the rebels wanted to "raise their flag" over Debaltseve and Mariupol before the ceasefire kicked in.
Ukraine's military said on Saturday morning that seven service personnel had been killed and 23 wounded over the past 24 hours.
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