Ukraine's military said 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 cease-fire, 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.

At around 07:00 GMT a 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 cease-fire was being observed in general. "Shelling [by rebels] has been not regular and [has been] localized," he said.

Separatist news service DAN quoted a senior rebel commander as saying rebels had opened fire in defense, after Ukrainian forces had violated the cease-fire around Debaltseve.

"Units were forced to open fire in response," Eduard Basurin was quoted as saying.