Pro-Russian rebels pounded Ukrainian troops near Debaltseve on 16 February, and Kiev said it would not pull back heavy guns while a truce was being violated, leaving a European-brokered peace deal on the verge of collapse a day after it took effect.
Fighting had subsided in many parts of eastern Ukraine after a ceasefire came into force from February 15, under a deal reached last week in marathon talks involving the leaders of France, Germany, Russia and Ukraine.
But the truce appears to have been stillborn near Debaltseve, in the part of the front where the most intensive fighting has taken place in recent weeks.
Rebels announced hours after it came into effect that they had no intention of observing the ceasefire at the town of Debaltseve, where they have been advancing since January and now have a Ukrainian unit all-but encircled.
Reporters near the front said Debaltseve was being relentlessly bombarded with artillery. At least six tanks as well as armoured personnel carriers and artillery could be seen in woods near Vuhlehirsk, which the rebels captured a week ago.
Military trucks headed along the main road in the direction of the town to regular bursts of shelling and firing of Grad rockets and machine guns.
Kiev said its forces were shelled more than 100 times after the truce took effect. Military spokesman Andriy Lysenko said government troops could not pull back their own heavy weapons, as set out in the agreement reached in the Belarussian capital Minsk, without a ceasefire that held.
Agreement on the truce raised hopes of ending a conflict that has killed more than 5,000 people.