Ukraine's military said on Monday it could not start withdrawing heavy weapons from the front line in the east as required under a tenuous ceasefire because pro-Russian separatists who advanced last week were still attacking its positions.
A truce to end fighting that has killed more than 5,600 people appeared stillborn last week after rebels ignored it to capture the strategic town of Debaltseve in a punishing defeat for Kiev.
Nevertheless, the peace deal's European sponsors still hold out hope it can be salvaged, now that the Moscow-backed separatists have achieved that objective.
Kiev says it fears the rebels, backed by reinforcements of Russian troops, are planning to advance deeper into territory the Kremlin calls "New Russia". Moscow denies aiding the rebels.
Fighting has diminished since Kiev's forces abandoned Debaltseve in defeat last Wednesday, and there were hopeful signs for the truce over the weekend, with an overnight exchange of around 200 prisoners late on Saturday and an agreement on Sunday to begin pulling back artillery from the front.
But Kiev said on Monday that it still could not start the artillery withdrawal.
"Given that the positions of Ukrainian servicemen continue to be shelled, there can not yet be any talk of pulling back weapons," spokesman Vladislav Seleznyov said in a televised briefing.
Anatoly Stelmakh, another military spokesman, said rebel forces had attacked the village of Shyrokyne overnight, along the coast on the road to Mariupol, a port of half a million that Kiev fears could be the next big rebel target.