Ukraine's military said on Sunday pro-Russian rebels had shelled Ukrainian positions ten times since a ceasefire came into effect at midnight, but that the truce was in general being observed.
The ceasefire, negotiated in four-power talks on Thursday, foresees the creation of a neutral buffer zone and withdrawal of heavy weapons responsible for many of the 5,000 deaths in a conflict that has caused the worst crisis in Russia-West relations since the Cold War a generation ago.
"As of the morning of February 15, there have been ten shelling attacks on our positions by rebels - mainly in the area of Debaltseve," spokesman Anatoly Stelmach said by telephone.
At around 0700 GMT a Reuters witness in east Ukraine heard the sound of heavy artillery fire coming from the direction of Debaltseve, a key transport hub where government forces have been hard pressed by encircling separatists.
Another military spokesman, Vladyslav Seleznyov, said the ceasefire was being observed in general. "Shelling (by rebels) has been not regular and (has been) localised," he said.
Separatist news service DAN quoted a senior rebel commander as saying rebels had opened fire in defence, after Ukrainian forces had violated the ceasefire around Debaltseve.
"Units were forced to open fire in response," Eduard Basurin was quoted as saying.