Ukrainian President Poroshenko on Wednesday called for European peacekeepers to enforce a shattered ceasefire deal in east Ukraine after a flashpoint strategic town fell to a fierce assault by pro-Russian rebels.

Some 2,500 exhausted government troops retreated from Debaltseve -- a key railway hub linking the main separatist-held cities of Donetsk and Lugansk -- after rebels ignored a supposed truce to seize control of the town.

The loss was a bitter blow to the Ukrainian army and left a last-ditch peace plan hammered out by the leaders of Ukraine, Russia, Germany and France hanging by a thread.

Ukraine's National Security and Defence Council approved a decision to invite UN-mandated peacekeepers into the country to monitor the frontline with the rebels and Ukraine's porous border with Russia.

"We see the best format would be a police mission from the European Union. We are sure this would be the most effective and best guarantee for security," Poroshenko said, adding that he hoped consultations on the force would start quickly once the decision is approved by parliament.

Ukraine and the US have accused Russia of piling troops and weapons in to spearhead the devastating assault on Debaltseve. Moscow denies the claims.

Poroshenko said six soldiers were killed and more than 100 wounded in what he earlier described as "a planned and organised withdrawal" he ordered from Debaltseve.

But haggard soldiers seen arriving in the neighbouring town of Artemivsk on tanks and other vehicles, or on foot, contested that characterisation.

"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.

- Soldiers taken prisoner -

There was no official casualty toll from the ferocious street-to-street battles that had taken place in the town since the rebels stormed it on Tuesday, but the bodies of at least 13 soldiers were seen delivered to the local morgue.

A row of makeshift coffins stood in the snow outside waiting to transport the corpses, still in camouflage uniform.

A spokesman for the rebels' military, Eduard Basurin, said the town was "completely under the control" of the insurgents, with just "scattered" pockets of resistance that were being neutralised.

Basurin claimed more than 300 government soldiers had been taken prisoner. Ukrainian officials conceded some were in rebel hands but would not say how many.

Amnesty International expressed concern about the treatment of the prisoners, noting evidence of brutality by both sides towards captives.

The situation in Debaltseve -- and the status of an estimated 5,000 civilians trapped there -- was impossible to verify. Journalists and monitors from the Organization for Security and Cooperation in Europe (OSCE) were unable to get into the town.

- Blow to truce -

Debaltseve's fall dealt a heavy blow to the European-brokered truce agreed by all sides in the Belarus capital Minsk last week and endorsed by the UN Security Council on Tuesday.

The ceasefire was meant to be the first step towards quelling a conflict that has killed more than 5,600 people since erupting in April 2014.

France has insisted the truce is "not dead", a view echoed by the US State Department on Wednesday, pointing to reports of the withdrawal of certain types of heavy weapons in Donetsk and Lugansk.

US State Department spokeswoman Jen Psaki added that Secretary of State John Kerry had spoken with Russian Foreign Minister Sergei Lavrov and pressed him "to stop Russian and separatist attacks on Ukrainian positions in Debaltseve and other violations of the ceasefire."

No details had yet emerged early Thursday of a planned phone call between German Chancellor Angela Merkel, French President Francois Hollande, Russian President Vladimir Putin and Poroshenko.

Ukraine and Western countries blame the relentless violence 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 playing a double game of insincere diplomacy and covert support to the separatists to keep Ukraine, a former Soviet republic, destabilised and stymied in its ambitions to edge closer towards the EU and NATO.

- Russian denial -

British Defence Secretary Michael Fallon warned Wednesday that NATO should be prepared for "any kind of aggression from Russia" to Baltic members of NATO -- eurozone members Estonia, Latvia and Lithuania -- using the tactics it is accused of employing in Ukraine.

Fallon referred to the interception of Russian bombers flying close to British airspace, and Russia's detention of an Estonian policeman that sent tensions soaring between Moscow and Tallinn, describing the threat to the eastern states as a "very real and present danger" in comments to British media.

NATO chief Jens Stoltenberg earlier called on Russia to "withdraw all its forces from eastern Ukraine, to stop its support for separatists and to respect the Minsk agreement".

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.

Russia denies involvement in the insurgency, though President Vladimir Putin had urged the surrender of the Ukrainian troops in Debaltseve before their withdrawal.