The rebels have advanced far past the line of an earlier ceasefire deal, agreed in September, and the new accord appears to envisage them withdrawing their guns around 75 km, to take them back behind it, while Ukrainian guns would move 25 km back.
Thursday's four-power accord also prescribed constitutional reform to give eastern Ukraine, where many Russian speakers live, more autonomy. Kiev has made clear it rejects independence for the "People's Republics" the rebels have declared.
Tatyana Demchenko, deputy commander of the rebel unit in the town of Horlivka, said she had little faith the ceasefire would hold. "They'll shoot at us and we have to remain silent? Militias may receive the order not to open fire, but what - we sit and die in shelling? If they don't shoot, we won't," she said, holding two grenades in her hands.
Ukrainian President Petro Poroshenko said the country had reached an important crossroads between war and peace.
"Either the enemy stops shooting and embarks on de-escalation ...or the enemy escalates the conflict for us and for Europe and the whole world," he said at a ceremony for border guards shown on television.
Seven Ukrainian service personnel have been killed and 23 wounded in fighting in the past 24 hours, Lysenko said.
The Group of Seven industrialised countries issued a statement late on Friday calling on all sides to refrain from actions that would hinder the start of the ceasefire. It said G7 countries were ready to take "appropriate measures" against those who violate the agreement, an apparent threat of more punitive economic sanctions against Russia shortly.
The deal, sealed by the leaders of Germany and France after 16 hours of all-night talks in Minsk, capital of Belarus, with the Russian and Ukrainian presidents, was quickly overshadowed by escalating bloodshed on the ground.