[KIEV] The leaders of Ukraine, Russia, Germany and France on Thursday jointly urged a tattered truce in east Ukraine be restored and condemned breaches in recent days, according to official accounts of a four-way telephone call.
Ukrainian President Petro 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.
Mr 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".
The German statement said Russian President Vladimir Putin promised to persuade the rebels to carry out a prisoner exchange.
Mr Poroshenko, Mr Putin, German Chancellor Angela Merkel and French President Francois Hollande all agreed that Organization for Security and Cooperation in Europe observers tasked with monitoring the ceasefire should be allowed to do their job unhindered.
The OSCE mission in Ukraine has complained for days that its team has been prevented by the rebels from entering Debaltseve, where thousands of Ukrainian troops were forced Wednesday to flee pro-Russian rebels who launched an assault in violation of the ceasefire.
The French presidency statement said OSCE representatives would meet the warring parties to "quickly put in place" the measures agreed last week in the Belarus capital Minsk.
It said "the four leaders called for the implementation of the full package of measures agreed in Minsk" including a full ceasefire, withdrawal of heavy weapons and the release of prisoners.
The statement said that foreign ministers from the four countries would speak later on Thursday to define how the ceasefire was being implemented. A French diplomatic source also told AFP there "will be a meeting, maybe in Paris, in coming days".
Mr Poroshenko's call for international "guarantees" in the event of more truce violations came on the heels of him calling for international peacekeepers to be deployed in east Ukraine.
He reiterated the appeal in the four-way call, saying talks should be held on the possibility of putting peacekeepers on the frontline, and along the porous Russia-Ukraine border.
Russia and the rebels have expressed strident opposition to that idea, claiming it would "destroy" the truce.
Ukraine and the West accuse Russia of cynically engaging in peace talks while covertly deploying soldiers and military assistance to the pro-Moscow separatists, who are gaining territory. The Kremlin denies the allegations.