Ukraine crisis: Battle rages for Debaltseve despite truce
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."
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.
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.
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.
- 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.
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.