Ukrainian forces lost five soldiers on Friday in a sudden resurgence of clashes ahead of peace talks aimed at ending the separatist conflict and mending East-West ties.
The toll was the highest since Kiev and Russian-backed militias struck a December 9 truce designed to reinforce a tenuous September agreement that was followed by at least 1,300 more deaths.
Last week's breakthrough was meant to set the stage for comprehensive talks that Ukranian President Petro Poroshenko had hoped to hold on Sunday with the help of European and Russian envoys in the Belarussian capital Minsk.
But a top rebel said the insurgents would only be ready by Monday -- a point underscoring the types of small squabbles that have hindered political progress throughout the eight-month war.
Separatists and the new leaders in Kiev who are trying to fold their ex-Soviet republic into the West began Skype video talks on Friday designed to nail down a final date.
"We proposed December 22 because it suits us better for technical reasons," separatist negotiator Denis Pushilin told AFP by telephone.
French President Francois Hollande -- who along with Poroshenko had joined two conference calls between German Chancellor Angela Merkel and Russia's Vladimir Putin in the past week -- also said the meeting would "happen on Sunday or Monday".
The four are expected to consult each other again over the next couple of days.
The scale of the fighting has subsided with the onset of winter and heavy snows that make progress across the war-scarred fields and muddied roads impossible.
All sides are now busy looking for ways to ensure that millions of civilians who have been unable to flee the artillery shelling and rocket fire make it safely through the winter in apartments with little to no water or heat.
The United Nations believes the daily battles have killed more than 4,700 people and driven nearly a million from their homes.
Its children's fund UNICEF said on Friday that "tens of thousands" of youth still lived in areas engulfed by violence.
"The situation for more than 1.7 million children affected by the conflict remains extremely serious," the UN Children's Rights and Emergency Relief Organisation said.
Any peace agreement is likely to include a requirement for fighters on both sides to let through humanitarian convoys they fear may be used to smuggle in weapons to their adversaries.
German Foreign Minister Frank-Walter Steinmeier said it was essential for the new agreement to set up a buffer zone that sets the initial boundaries of areas overseen by the rebels within a unified Ukraine.
Steinmeier added after talks in Kiev with Poroshenko and Prime Minister Arseniy Yatsenyuk that the sides must also agree to swap their remaining prisoners and "resolve humanitarian relief issues".
Poroshenko had been expected to urge Steinmeier to hold strong on sanctions against Russia that some EU member nations think should be gradually eased in the coming months.
EU leaders on Thursday slapped sanctions against Russian-administered Crimea they had approved at an earlier meeting but took no additional steps against Russia itself.
The White House on Thursday said US President Barack Obama also did not intend to impose new punitive measures on Russia despite signing an act approved by Congress that allows him to do so at any point.
Obama further stressed that Washington -- like Brussels -- was ready to lift its restrictions if Russia de-escalated the conflict by paying respect to "Ukraine's sovereignty and territorial integrity".
But Russian Foreign Minister Sergei Lavrov still warned the White House that legislation "threatening new sanctions against Russia could undermine the possibility of normal cooperation between our countries for a long time."