Ukraine conflict: Poroshenko calls for UN peacekeepers
President Petro Poroshenko has called for UN peacekeepers to be deployed to eastern Ukraine to enforce a ceasefire.
At an emergency security meeting, he said such a force would help guarantee security "in a situation where the promise of peace is not being kept".
His comments came after Ukrainian troops withdrew from the strategic city of Debaltseve after intense fighting.
The rebel advance on Debaltseve, which came in spite of the ceasefire agreed last week, has been widely condemned.
Mr Poroshenko's request was approved by Ukraine's national security and defence council at the emergency meeting on Wednesday night.
"The issue was discussed and a decision has been taken to appeal to the UN and the EU concerning the setting up in Ukraine of a peacekeeping and security operation," council secretary Olexander Turchynov told reporters.
The move came after President Poroshenko said almost 2,500 Ukrainian soldiers had withdrawn from Debaltseve on Wednesday.
He said the withdrawal had been organised, but that at least six soldiers were killed and more than 100 wounded.
Earlier, a senior Ukrainian military official said 22 Ukrainian soldiers had died in Debaltseve over the past three days. Rebel claims of a much higher figure have been dismissed by the government.
The ceasefire, which came into effect on Sunday, has been broadly observed elsewhere in eastern Ukraine and some heavy weaponry is said to have been withdrawn by both sides.
However, staff from the OSCE security group that is monitoring the ceasefire have been unable to reach Debaltseve.
Rebel spokesman Eduard Basurin said the town was now "completely under the control" of the separatists, with just "scattered" pockets of resistance that were being "neutralised".
He claimed that more than 300 government soldiers had been taken prisoner. Ukraine's government admitted some troops were being held.
At the scene: Paul Adams, BBC News, eastern Ukraine
All Wednesday, the road out of Debaltseve into government-held territory thundered to the sound of retreating armour - tanks and troop carriers full of exhausted, sometimes defiant soldiers.
In a bus by the side of the road, I found one Yuri slumped on his seat, across the aisle from a sleeping colleague.
He said the situation had become increasingly dire and individual units had taken their own decisions to leave. They were running out of ammunition and in danger of being surrounded, he told me.
He blamed Russian President Vladimir Putin for deceiving everyone about the ceasefire. We know him well by now, he said. When he says something is guaranteed, that means there's some kind of trap coming up.
In nearby fields, mortars and multiple rocket launchers fired back at the rebels - providing cover for those still trying to leave Debaltseve.
President Poroshenko says the withdrawal was planned and organised, but on the ground, it looked like a hasty retreat in the face of overwhelming odds.
Nato chief Jens Stoltenberg said the rebels' offensive had put the wider peace agreement at risk.
The White House said both the rebels and Russia had failed to live up to the terms of the agreement signed in the Belarusian capital Minsk last week.
But Russian Foreign Minister Sergei Lavrov insisted the rebels' actions in Debaltseve had not violated the ceasefire because it was a rebel-held city at the time of the agreement.
The BBC's Sarah Rainsford in Moscow said the feeling there is that a lull in the fighting may now be possible - rather than an immediate push for more territory by the rebels.
Mr Poroshenko was due to discuss the situation in a phone call with Russian President Vladimir Putin and the leaders of France and Germany.
Correspondents say the loss of Debaltseve is a significant setback for Ukraine because of its strategic position as a transport hub between the rebel-held cities of Donetsk and Luhansk.
Most of its 25,000 population has been evacuated but about 5,000 civilians are still believed to be in the town.
One rebel commander there told the BBC that conditions were dire, with no electricity and a shortage of food and water.
Fighting began in eastern Ukraine in April, a month after Russia annexed the Crimea peninsula.
The UN says more than 5,600 people have been killed, but there are fears the actual death toll could be much higher.
Ukraine's pro-Western government says Russia is supporting the separatists with troops and weapons, but the Kremlin has consistently denied this.
- Ceasefire from 00:01 on 15 February (22:01 GMT 14 February)
- Heavy weapons to be withdrawn, beginning on 16 February and completed in two weeks - beyond a buffer zone behind the current front line for Ukrainian forces and behind the September front line for separatist forces
- All prisoners to be released; amnesty for those involved in fighting
- Withdrawal of all foreign troops and weapons from Ukrainian territory. Disarmament of all illegal groups
- Ukraine to allow resumption of normal life in rebel areas, by lifting restrictions
- Constitutional reform to enable decentralisation for rebel regions by the end of 2015
- Ukraine to control border with Russia if conditions met by the end of 2015