Pro-Russian rebels on Saturday warned that any attempts by Kiev forces to move out of the battleground town Debaltseve after the start of a ceasefire would be considered an aggression.
Separatist forces have all but encircled government troops inside the strategic railway hub, roughly mid-way between insurgent capital Donetsk and Lugansk.
Fierce fighting raged around Debaltseve on Saturday in the hours before the 2200 GMT start of a truce intended to be a first step in a peace plan to end 10 months of conflict.
Kiev denies rebel claims that thousands of government troops have been trapped in the town.
Donetsk rebel leader Alexander Zakharchenko -- considered by the West to be a Kremlin puppet -- on Saturday warned that any attempt by Ukraine to move its troops out of Debaltseve after the ceasefire would be seen as a violation of the truce.  
"These attempts will be stopped by us and the enemy will be destroyed," Zakharchenko said.
Ukraine and the West accused Russia of piling in heavy weapons to spearhead the frantic separatist onslaught for the vital position.
The threat highlights the fragility of the looming truce, and could be used by the separatists to justify a return to fighting.
Kremlin spokesman Dmitry Peskov echoed the rebel warning.
"Ukrainian army units that are surrounded even after the start of the ceasefire will naturally try to break out and thus break the truce regime," Peskov was quoted as saying by Russian news agency Interfax.
"There is a potential for such a danger."
Kiev, however, lashed out at the rebel leaders, accusing Zakharchenko of essentially announcing that his forces would not respect the ceasefire.
"This is just more proof of the undisguised attempts by Russia and the terrorists it controls to scuttle the agreements for a ceasefire and peaceful resolution," Ukraine's national security council said in a statement.