The truce envisages the creation of a neutral "buffer zone" and withdrawal of the heavy weapons responsible for many of the 5,000 casualties in the conflict that broke out almost a year ago.
"Ahead of midnight, rebels are trying to complete tactically important plans to enlarge the territory under their control, primarily in the direction of Debaltseve," spokesman Andriy Lysenko said at a daily televised briefing in Kiev.
Debaltseve, a strategic transport hub northeast of the rebel-controlled city of Donetsk, has been the focus of some of the fiercest fighting in recent weeks.
Lysenko said separatists were receiving support from fighters and military equipment crossing the border from Russia into Ukraine. Moscow denies supplying the rebels with arms and troops although Western officials cite overwhelming evidence to the contrary.
The rebels have advanced far past the line of an earlier ceasefire deal, agreed in September, and the new accord appears to envisage them pulling their guns back around 75 km, to take them back behind it, while Ukrainian guns would move 25 km back.
Seven Ukrainian service personnel have been killed and 23 wounded in fighting in the past 24 hours, Lysenko said.
