The tense negotiations entered their eleventh hour after opening Wednesday evening with a brief handshake between arch-foes Ukrainian President Petro Poroshenko and Russian leader Vladimir Putin, who were meeting for the first time since October.
Tortuous talks in Minsk between the leaders of Ukraine, Russia, Germany and France dragged towards dawn Thursday as they tussled over a plan to end 10 months of fighting in Ukraine.
The crunch four-way meeting with German Chancellor Angela Merkel and French President Francois Hollande in the Belarussian capital was the climax of a frantic European diplomatic drive aimed at stopping the worst East-West crisis since the end of the Cold War from escalating.
Underscoring the urgency, the number of those reported killed in the hours before the make-or-break talks rose to at least 49 as rebels said another civilian was killed when a hospital in their bastion Donetsk was shelled as the leaders met, following the earlier deaths of 16 people in a devastating rocket attack.
"Today the peace process for Ukraine is all about Minsk and I hope that the meeting will fulfil our best expectations," Poroshenko told host Belarussian leader Alexander Lukashenko ahead of the talks.
'Yawning but still arguing'
By 6 am local time (0300 GMT) the marathon negotiations passed the ten-hour mark and the four leaders remained shut in an ornate meeting room in Minsk's opulent presidential palace without their advisors.
"Everybody is yawning but they are still arguing," a source close to one of the delegations told AFP.
France's Hollande had said prior to the meeting in Minsk that he and Merkel would "try everything right to the end" to try to get something from the last-ditch meeting.
Earlier, a senior Ukrainian diplomat told AFP on condition of anonymity that the talks were making "progress" but also proving "very hard".
Another source with knowledge of the discussions had said the leaders hoped to sign a joint statement calling for the fulfilment of an earlier failed peace plan signed by Kiev and the rebels in September.
Separatist negotiators meanwhile met Wednesday elsewhere in Minsk to agree how to implement previous truce deals with representatives from Kiev, Moscow and mediators from the Organization for Security and Co-operation in Europe.
The most pressing element is the need to agree an immediate ceasefire between the two sides that would see an end to the surging fighting that has killed hundreds of civilians in recent weeks.
A key sticking point is whether a new deal will extend rebel control over 500 square kilometres (200 square miles) of territory seized over the past month.
Western diplomats warn that the sides also remain deadlocked over other key issues including how Ukraine can shore up a rebel-controlled 400 kilometres (250 miles) stretch of its border with Russia, across which it accuses Moscow of pouring arms and fighters.
Moscow is also pushing for the separatist-held territories to be granted a large degree of autonomy, but Kiev only says that it is willing to decentralise some powers.
Poroshenko warned before the talks that he would introduce martial law throughout the country if they fail to stop a war that has already claimed more than 5,300 lives.
Martial law would mark a significant escalation of the crisis, freeing up military resources for the fight in the east but also likely leading to the severance of foreign investment for cash-strapped Ukraine, including a vital IMF loan.
At the meeting in the marble-floored palace the European leaders and pro-Western Poroshenko put on a united front as they smiled for cameras and chatted amiably.
Poroshenko had said that he, Hollande and Merkel would speak "with one voice" to Putin, whom they accuse of backing the rebellion.
Poroshenko is scheduled to brief a European Union summit in Brussels on Thursday.
European Union foreign policy chief Federica Mogherini said the talks would be a "turning point for good or bad", while Russia -- accused by the West of fomenting the war by pouring troops and weapons across the border -- voiced optimism.
Germany said the meeting, the most intensive international push for an end to the bloodshed in the east of the former Soviet state, offered a "glimmer of hope, nothing more".
If the Minsk talks fail, US President Barack Obama has warned that Washington may decide to start providing lethal weapons to Ukraine, a step many European leaders oppose for fear of getting drawn into open conflict with Russia.
The bloodletting in eastern Ukraine has been relentless in recent weeks as the rebels have pushed deeper into government-held territory and Kiev forces have counter-attacked.
As the leaders converged on Minsk, fighting raged on the ground with both sides trying to strengthen their hand at the negotiating table.
Insurgents have been battling for weeks to take the rail hub of Debaltseve, while Ukrainian forces on Tuesday captured ground around the port city of Mariupol.
By Max Delany (AFP)