The peace process in Ukraine is being "threatened" by pro-Russian separatists who have virtually encircled Ukrainian troops in the key battleground town of Debaltseve, President Petro Poroshenko said shortly after the start of an agreed ceasefire.
"Unfortunately, the peace process is threatened, the rebels will use Debaltseve to undermine the ceasefire," Mr Poroshenko said during a meeting with the army chief of staff broadcast live on television, after ordering the military to implement the ceasefire from midnight (10pm Irish time).
"As supreme commander of the armed forces, I want peace," he added.
A surge in fighting around the strategic railway hub of Debaltseve in the hours before the ceasefire cast doubt on whether the truce will be respected.
The ceasefire is the first step in a fragile peace plan aimed at ending 10 months of conflict in the east of the country.
Under the terms of the deal, agreed on Thursday after marathon talks in Minsk between the leaders of Ukraine, Russia, Germany and France, the two warring sides have two days from the start of the truce to start pulling back heavy weapons from the frontline.
The last-ditch peace plan is seen as the best hope of ending the violence that has claimed at least 5,480 lives since April but scepticism remains high after the collapse of a similar previous deal.
Ferocious fighting raged in the hours before the ceasefire began, with Ukraine and the United States accusing Russia of piling in weapons to fuel a rebel onslaught to grab territory.
Russian President Vladimir Putin reaffirmed that the ceasefire in Ukraine must be respected.
In a joint phone call, German Chancellor Angela Merkel, French President Francois Hollande and Mr Putin all confirmed the need for the ceasefire to be effective, French officials said.
Earlier, both Ukraine and pro-Russian rebels pledged to respect the ceasefire which came into force after fierce fighting plunged the truce into doubt.
Seven Ukrainian service personnel were killed and 23 wounded in fighting in the past 24 hours, Ukrainian officials said.
Mr Poroshenko made the pledge in a phone conversation with Ms Merkel and Mr Hollande, the French presidency said in a statement.
On the other side, rebel leader Alexander Zakharchenko - who Ukraine and the West see as a Kremlin puppet - signed an order commanding his troops to observe an "immediate and total ceasefire" at the agreed time.
The order did, however, give the rebel forces the right to respond to any perceived attacks "with all force and means".
