German government spokesman Steffen Seibert said on Thursday that the four leaders had agreed to stick to the ceasefire terms agreed at last week's summit in Minsk in a telephone conversation on Thursday.

Chancellor #Merkel, Pres. @fhollande, @poroshenko + #Putin agree on the phone to remain committed to #Minsk agreements. MT: @RegSprecher

— GermanForeignOffice (@GermanyDiplo) February 19, 2015

President Putin had promised to put pressure on the rebels to implement the ceasefire, withdraw heavy weapons and begin prisoner exchanges, Seibert said.

But Poroshenko accuses the pro-Russian rebels of having systematically and knowingly breached the terms of the ceasefire from the moment it come into effect.

Poroshenko slammed in particular the rebel storming of the key eastern town of Debaltseve this week and called for international "guarantees" should further violations happen, his office said.

Poroshenko stressed that a stalled withdrawal of heavy weapons from the frontline, agreed under the German- and French-mediated truce, could only happen once the ceasefire was deemed to be "comprehensive".

He also called for all prisoners to be released – "including those who had been captured near Debaltseve".

Six government soldiers were killed and more than 100 injured while thousands of troops withdrew on Wednesday morning from the vital railway junction that had been under siege by the rebels.

Poroschenko called on Wednesday for UN peacekeepers to be sent into conflict zones in the east of his country, after the Debaltseve withdrawal.

“I am pushing for us to discuss inviting a UN peace mission under a mandate from the UN Security Council,” he said at a meeting of his national security council.

“For us, the best format would be a policing mission from the European Union [EU].”

Ukraine is opposed to deploying blue-helmeted UN peacekeepers, rather than European troops, because it fears that a UN-led mission could become permanent.

But Russian President Vladimir Putin refuses to accept the idea of EU troops in Ukraine, arguing that western Europe is a party to the conflict.

Poroshenko said that he had already discussed the idea of sending in peacekeepers with Merkel, Hollande and Putin at last week's summit in Minsk.

At a party conference in Mecklenburg-Vorpommern, Merkel told her colleagues from the Christian Democratic Union (CDU) that “the federal government, European governments, Germany and France together won't let up in doing everything so that Ukraine can go its own way and has its territorial integrity.”

She accused Russia of breaching international law by annexing the Crimean peninsula, but said the EU would do everything “to make Russia our partner again.”

“We want to build a peaceful system in Europe with Russia, not against Russia,” she added.