Ukraine's rebels disavowed a new truce on Sunday hours after it took effect, saying it did not apply to the town where most fighting has taken place in recent weeks.
Guns fell abruptly silent at midnight across much of eastern Ukraine in line with the ceasefire agreement, reached after a week of marathon diplomacy led by France and Germany.
But pro-Russian rebels announced they would not observe the truce at Debaltseve, where Ukraine army forces have been encircled, Reuters reported.
"Of course we can open fire (on Debaltseve). It is our territory," Eduard Basurin, a senior rebel commander said. "The territory is internal, and internal is internal. But along the line of confrontation there is no shooting."
It was unclear what impact that disavowal would have across the battle zone, where the emphasis was mostly on ensuring the truce would stick. Both sides said their forces had stopped shooting and blamed what firing there was on the enemy.
Kremlin spokesman Dmitry Peskov said the truce must be implemented "unconditionally" as agreed on Thursday, but made no mention of whether Moscow believes the ceasefire applies to Debaltseve. He declined to comment on Basurin's remarks.
Ukrainian forces have for weeks been holding out in the town, which sits astride a railway junction in a pocket between the two main rebel strongholds. Elsewhere in eastern Ukraine the ceasefire was met by abrupt silence at midnight. According to reports, artillery bombardment in Donetsk halted and no firing was heard overnight, after intense final hours before the ceasefire when shells had exploded every few seconds.
The Ukrainian military said on Sunday morning that the ceasefire was being "generally observed". Its forces had been shelled 10 times since the truce took effect in "localized" incidents. Nine of its soldiers were killed on Saturday but none since the truce took effect, a spokesman said on Sunday morning.
