18 February 2015
Last updated at 14:58
The Ukrainian president says his forces are making an “organised” withdrawal from the embattled town of Debaltseve.
Petro Poroshenko said 80% of Ukraine’s troops left on Wednesday morning after several days of fierce fighting.
Russia said Ukrainian forces had tried to fight their way out of the city after being encircled but Mr Poroshenko insisted they were never surrounded.
The rebel advance on Debaltseve, which came in spite of the recent ceasefire agreement, has been widely condemned.
Nato chief Jens Stoltenberg said the rebels’ offensive had put the wider peace agreement at risk and urged Russia to “use all its influence on the separatists to make them respect the ceasefire”.
He also called on Moscow to withdraw its forces from Ukraine, saying Russian troops, artillery and air defence units were still active in the country.
Earlier, Russian Foreign Minister Sergei Lavrov insisted the rebels’ actions in Debaltseve had not violated the ceasefire because it was a rebel-held city when the peace agreement was signed last week.
He urged rebels to provide troops who surrendered with food and clothes and said he hoped the situation in the city would “not be used to find a pretext to actually undermine [the agreement]”.
Eyewitnesses saw dozens of tanks and columns of weary Ukrainian troops retreating from Debaltseve on Wednesday.
Russia’s state-controlled Channel One TV showed footage of what it said were rebels raising their flag on top of a high-rise building in the city.
Mr Poroshenko said in a statement: “Debaltseve was under our control, there was no encirclement, and our troops left the area in a planned and organized manner.”
He also called for “a firm reaction from the world to Russia’s brutal violation of the Minsk agreements, the ceasefire regime and the withdrawal of heavy weaponry”.
Mr Poroshenko said he was heading to the front line to meet the soldiers who had left Debaltseve, saying it would be an honour to shake hands with “Ukrainian heroes”.
The government in Kiev has admitted that some soldiers were taken prisoner in Debaltseve, but gave no details on how many were seized. Rebels earlier said hundreds of Ukrainian soldiers had given themselves up but the president denied this.
The withdrawal comes after Russian President Vladimir Putin urged Ukraine’s troops in Debaltseve to surrender.
Mr Putin is due to speak by telephone later to German Chancellor Angela Merkel, Frence President Francois Hollande and Mr Poroshenko, according to the French government.
Spokesman Stephane Le Foll insisted the agreement announced last week by the four leaders to end the fighting in eastern Ukraine was not dead, and that progress had been made.
Meanwhile, a spokesman for Chancellor Merkel condemned the rebel action in Debaltseve, saying it was a clear violation of the truce but that it was too early to determine whether that had failed.
Analysis: David Stern, BBC News, Kiev
The rapidly deteriorating situation inside Debaltseve and its possible fall to Russian-backed forces – despite a ceasefire brokered specifically to avoid this scenario – raises a number of questions but one in particular: What were Angela Merkel and Francois Hollande thinking?
Did they receive assurances that the insurgents would observe the ceasefire? Or was the fate of the strategic railroad junction under question even during the Minsk negotiations? If so, why did the Europeans proceed with an agreement that was unenforceable?
It’s entirely possible that the Minsk accords have done irreparable damage to the peace process and seriously weakened the position of President Petro Poroshenko. Ukrainian officials may very well be asking what other “exceptions” to the ceasefire will the rebels insist on?
Mrs Merkel and Mr Hollande have long insisted there’s no military solution to the war in eastern Ukraine. Now there may not be a diplomatic one either.
International observers monitoring the truce have been unable to enter Debaltseve.
The city has become a key prize for rebels and government forces, as it sits on a strategic railway line linking the rebel-held cities of Donetsk and Luhansk.
Most of its 25,000 population has been evacuated but about 7,000 civilians are still believed trapped by the fighting.
The ceasefire, which came into effect on Sunday, has been broadly observed elsewhere.
Rebel leaders in the so-called Donetsk People’s Republic said on Wednesday they had begun to withdraw heavy weaponry from the parts of the frontline where the ceasefire was holding.
The withdrawal was due to start no later than the second day after the truce came into effect and be completed within two weeks, creating buffer zones 50-140km (30-85 miles) wide.
The UN says more than 5,600 people have been killed in the conflict since April, 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.
British Prime Minister David Cameron has described Russia’s actions in Ukraine as “completely unacceptable” and warned of “financial consequences”.
This comes after US Vice-President Joe Biden said he “strongly condemned the violation of the ceasefire by separatist forces acting in concert with Russian forces, in and around the town of Debaltseve”.
He added: “If Russia continues to violate the Minsk agreements… the costs to Russia will rise.”
- 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