ARTEMIVSK, Ukraine - Ukrainian troops pulled out of a besieged flashpoint eastern town on Wednesday after it was stormed by pro-Russian rebels in what the EU said was a “clear violation” of an internationally-backed truce.

The retreat from Debaltseve - a strategic railway hub sandwiched between the main rebel-held cities of Donetsk and Lugansk - was a serious defeat for Ukrainian President Petro Poroshenko, who came to office vowing to crush the separatist uprising.

Poroshenko, who flew to the front as the pullout was under way, described the exodus as “a planned and organised withdrawal”. He said 80 percent of the soldiers deployed in the town had left and spoke only of “30 wounded” among them.

Haggard soldiers in grubby clothes seen arriving in Debaltseve’s neighbouring town of Artemivsk on dozens of tanks and other vehicles, or on foot, contradicted Poroshenko’s words.

“We didn’t hear anything about an order to pull out. We only found out about it when our heavy armour started leaving,” one soldier told AFP, declining to give his name or unit. “We should have pulled out earlier,” another said bitterly. The bodies of at least 13 soldiers were delivered to a morgue in the nearby town of Artemivsk.

A row of makeshift coffins stood in the snow outside waiting to transport the corpses, still in camouflage uniform. Debaltseve’s fall dealt a heavy blow to the European-brokered truce agreed by all sides last week in a bid to quell a 10-month conflict that has killed more than 5,600 people.

German Chancellor Angela Merkel’s spokesman said in Berlin that the rebel offensive on Debaltseve had put a “heavy strain” on hopes for peace. But France insisted the truce it and Germany helped broker last week in the Belarus capital Minsk “was not dead”.

Merkel, French President Francois Hollande, Russian President Vladimir Putin and Poroshenko were to speak by telephone later Wednesday.

Western countries blame the relentless violence in Ukraine on Russia, which annexed Crimea in March last year and is accused of sending in troops and heavy weapons across the border to back the eastern insurgency.

They say Moscow is cynically playing with the truce - which won unanimous backing from the UN Security Council on Tuesday - to keep Ukraine destabilised.

“The actions by the Russia-backed separatists in Debaltseve are a clear violation of the ceasefire,” EU foreign policy chief Federica Mogherini said in a statement.

NATO chief Jens Stoltenberg called on Russia to “withdraw all its forces from eastern Ukraine, to stop its support for separatists and to respect the Minsk agreement”. “I am deeply concerned by the deteriorating situation in and around Debaltseve,” he said in Latvia.

Putin, who repeatedly denies directly helping the insurgency, on Tuesday urged the Ukrainian soldiers in Debaltseve to surrender. “Obviously it’s bad to lose,” Putin said during a visit to Hungary. “But life is life and it still goes on.”

A rebel military spokesman, Eduard Basurin, told Russian television on Wednesday that more than 300 Ukrainian soldiers had given themselves up to the insurgents, the Interfax news agency reported.

A regional deputy police chief in Debaltseve, Ilya Kiva, told AFP by telephone that some soldiers were taken prisoner but would not say how many.

Amnesty International expressed concern about the treatment of prisoners taken in the Ukraine conflict, noting brutality by both sides.

An estimated 5,000 civilians are thought to have been trapped in the town, with little water or food in the harsh winter.

Journalists, and Organization for Security and Cooperation in Europe officials tasked with monitoring the ceasefire, have been barred from Debaltseve by the rebels and were unable to verify the situation inside.

The West has already imposed several rounds of sanctions on Russia for its perceived role in Ukraine, and the EU and Canada ratcheted their measures up further this week.

In Washington, the White House “strongly condemned the violation of the ceasefire by separatist forces acting in concert with Russian forces” and warned “the costs to Russia will rise” if the truce is further violated.

A worker at a hospital in town of Gorlivka near Debaltseve told AFP that several soldiers underwent emergency surgery there Wednesday. Occasional artillery blasts could be heard.

A Gorlivka municipal councillor, Tatiana Dementchenko, said “the shelling hasn’t stopped” in the area since it supposedly came into effect on Sunday. “We really hope that will all cease very soon. We’ve lost so many men, so many civilians,” she said.