Media playback is unsupported on your device Media caption The ceasefire is under "impossible strain", says Jonny Dymond 
 
 Fierce fighting is reported inside the key Ukrainian town of Debaltseve despite a ceasefire agreed last week. 
 
 Rebels say they have taken most of Debaltseve, a transport hub, but the government says it is still holding its positions. 
 
 International observers tasked with monitoring the ceasefire have been unable to enter the town. 
 
 Ukraine's president described rebel attempts to take the town as a "cynical attack" on the ceasefire. 
 
 "Today the world must stop the aggressor," Petro Poroshenko said in a statement posted on his website. 
 
 "I call on the permanent members of the UN Security Council to prevent further violation of fundamental principles and rules of the UN and the unleashing of a full-scale war in the very centre of Europe." 
 
 Speaking on a visit to Hungary, Russian President Vladimir Putin said he hoped the ceasefire agreements would be observed by both sides. 
 
 French Foreign Minister Laurent Fabius said "fundamental" parts of the ceasefire were not being respected, referring to the Debaltseve fighting and withdrawal of heavy weapons. 
 
 Earlier, both sides failed to begin the withdrawal, despite a Monday deadline agreed in the truce. 
 
 The two sides were given until two days after the latest ceasefire came into effect to start the pullout. 
 
 Analysis: Paul Adams, BBC News, Kramatorsk 
 
 The wealth of claim and counter-claim around Debaltseve speaks volumes. It's hard to confirm any of today's stories. 
 
 Controversy surrounds the fate of dozens of government troops - rebel sources say they surrendered, while the army contends they were captured after running out of ammunition during an ambush. 
 
 The rebels say Debaltseve is not covered by the ceasefire agreement reached last week in Minsk and continue to insist that it's an "internal" matter. Between April and July last year, the town was in rebel hands. It sits astride the railway line linking two rebel strongholds, Donetsk and Luhansk. 
 
 Ukrainian TV has shown pictures of text messages sent to government soldiers in Debaltseve, allegedly from Russia. "Poroshenko and his generals have betrayed you," the messages read, referring to the Ukrainian president. "There's no need for you to die for them." 
 
 Why is conflict so violent? 
 
 Who benefits from ceasefire deal? 
 
 Explaining the conflict in maps 
 
 Sources in the self-proclaimed Donetsk People's Republic (DPR) said Debaltseve police station and railway station had been taken, and at least 80% of the city was under rebel control. 
 
 According to later reports, the city's military HQ - where many government troops are based - has also been surrounded. 
 
 Image copyright Reuters Image caption Government artillery outside Debaltseve. Neither side has withdrawn its heavy weaponry as yet 
 
 Image copyright Reuters Image caption This car carrying journalists and a wounded driver fled shelling near Debaltseve on Tuesday 
 
 Image copyright AP Image caption Ukraine's military says it won't withdraw heavy weapons until it sees rebels do the same 
 
 DPR spokesman Eduard Basurin said government forces had stepped up their bombardment and were heading towards the rebel-held town of Lohvynove in an attempt to break through to Debaltseve. 
 
 He added that the rebels intended to withdraw heavy weapons from "quiet sectors" of the front line, but gave no time-frame. 
 
 The rebels said that up to 300 Ukrainian troops in Debaltseve had surrendered, and Russian TV showed footage of what it said were 72 captured soldiers. The Ukrainian government said a group had been taken prisoner after an ambush but denied large-scale surrenders. 
 
 The Ukrainian military said there was intense fighting in the streets and confirmed that the rebels were in control of parts of the city. 
 
 "Our troops are holding their positions, and they are well within their rights to return fire and hold the positions that they have held for several months," Interfax quoted spokesman Andriy Lysenko as saying. 
 
 'Safe corridor' 
 
 A National Guard source in the area told the BBC that government forces had regained control of the supply route, enabling them to get food and medicine into the town and wounded soldiers out. 
 
 Meanwhile, Russia's LifeNews website reported that rebel leader Alexander Zakharchenko had been lightly wounded in the leg during the fighting in Debaltseve and had been evacuated from the city. 
 
 Rebels have offered Ukrainian troops under siege there a safe corridor to leave. 
 
 Although Debaltseve has suffered weeks of artillery exchanges, correspondents say this is the first fierce fighting inside the town. 
 
 Most of Debaltseve's 25,000 population have been evacuated but about 7,000 civilians are still believed trapped by the fighting, according to Amnesty International. 
 
 The BBC's David Stern in Kiev says there is great concern about the humanitarian situation, with water now running out. 
 
 Minsk agreement: Key points 
 
 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 
 
 The ceasefire, which came into effect on Sunday, has been broadly observed but separatists insist the agreement does not apply in Debaltseve because they have the town almost surrounded. 
 
 Denis Pushilin, a spokesman for the Donetsk People's Republic, described Debaltseve as "internal territory" and said fighting for it was "a moral thing". 
 
 "We do not have the right [to stop fighting]," he told Reuters. 
 
 In the neighbouring Luhansk region, separatist leader Igor Plotnitsky said he had begun pulling back his tanks and artillery in line with the ceasefire agreement. His claim could not be independently verified. 
 
 Image copyright AP Image caption Ceasefire monitors have been unable to visit Debaltseve 
 
 Ukraine's pro-Western government says Russia is supporting the separatists with troops and weapons, but the Kremlin has consistently denied this. 
 
 Meanwhile, the leaders of Germany, Ukraine and Russia discussed the crisis in an overnight phone call. 
 
 Germany said they had agreed "concrete measures" for observers to have greater access, but gave no details. 
 
 Observers from the Organization for Security and Co-operation in Europe (OSCE), who are charged with monitoring the ceasefire, have been trying to reach Debaltseve after being denied access by pro-Russian rebels on Sunday. 
 
 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. 
 
 Officials say more than 5,400 people have been killed since the conflict erupted in eastern Ukraine in April, but the UN believes the actual death toll to be much higher. ||||| Media playback is unsupported on your device Media caption President Poroshenko met with troops who withdrew from Debaltseve 
 
 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 town 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]". 
 
 Media playback is unsupported on your device Media caption Footage of Debaltseve captured by a drone showed evidence of shelling 
 
 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 town. 
 
 Later rebel spokesman Eduard Basurin was quoted in Russian media as saying Debaltseve was fully under the control of separatists, although there were still "disparate groups of the enemy" in the southern part of town. 
 
 One rebel commander in the city told the BBC that conditions were dire, with no electricity and a shortage of food and water. He said rebels were sharing their rations with the remaining civilians. 
 
 'Brutal violation' 
 
 President Poroshenko said in a statement: "Debaltseve was under our control, there was no encirclement, and our troops left the area in a planned and organised manner." 
 
 He 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 visited the soldiers who had left Debaltseve in the town of Artemivsk on Wednesday. Earlier, he said it would be an honour to shake hands with "Ukrainian heroes". 
 
 A senior Ukrainian military official said 22 Ukrainian soldiers had died in Debaltseve over the past three days. 
 
 Earlier, an official at a morgue in Artemivsk said the bodies of 25 Ukrainian soldiers had been brought to the facility from Debaltseve but this has not been confirmed. 
 
 Rebels have claimed that hundreds of Ukrainian troops were killed in clashes around the city, but Mr Poroshenko denied this. 
 
 The government in Kiev admitted that that some soldiers were taken prisoner in Debaltseve, but gave no details on how many were seized. 
 
 At the scene: Paul Adams, BBC News, eastern Ukraine 
 
 Image copyright Reuters 
 
 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. 
 
 The withdrawal came after Russian President Vladimir Putin urged Ukraine's troops in Debaltseve to surrender. 
 
 Mr Putin is due to speak by telephone later on Wednesday to German Chancellor Angela Merkel, French 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. 
 
 Image copyright Reuters Image caption The Ukrainian troops appeared weary and dirty as they withdrew from the besieged city 
 
 Image copyright EPA Image caption Some 7,000 civilians are said to have been trapped in the Debaltseve area during recent fighting 
 
 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 and some rebel heavy weaponry was said to have been withdrawn. 
 
 The UN says more than 5,600 people have been killed in the conflict, but there are fears the actual death toll could be much higher. 
 
 Fighting began in eastern Ukraine in April, a month after Russia annexed the Crimea peninsula. 
 
 Ukraine's pro-Western government says Russia is supporting the separatists with troops and weapons, but the Kremlin has consistently denied this. 
 
 Minsk agreement: Key points
– A ceasefire in Ukraine didn't stop fighting in the rail hub of Debaltseve, which links two rebel strongholds. Now, government troops are withdrawing from the area in what the Wall Street Journal calls a big setback; Ukraine had previously said it would hold the town as it grappled with losses elsewhere, and the move may weaken President Petro Poroshenko while affirming the separatists' power, the Journal notes. Some 80% of units in the town have departed. "The priority must be saving the lives of the Ukrainian troops," said Russia's foreign minister; the BBC reports that he called on rebels to feed surrendering Ukraine troops.