A peace plan drawn up by France and Germany is "one of the last chances" to end the conflict in east Ukraine, French leader Francois Hollande says.
German Chancellor Angela Merkel said it was unclear if the plan would succeed, but it was "definitely worth trying".
Mr Hollande said the plan would include a demilitarised zone of 50-70km (31-44 miles) around the current front line.
The leaders are attempting to end the fighting in Ukraine between government troops and pro-Russia rebels.
Russia is accused of arming pro-Russian separatists in eastern Ukraine - claims it denies.
Mr Hollande and Mrs Merkel are due to discuss the peace plan with Russian President Vladimir Putin and Ukrainian leader Petro Poroshenko by telephone on Sunday.
Few details have emerged, but the plan is thought to be an attempt to revive a failed ceasefire deal signed in Minsk, in Belarus, in September. Since then, the rebels have seized more ground, raising alarm in Kiev and among Ukraine's backers.
The UN says fighting has left nearly 5,400 people dead since April, when the rebels seized a big swathe of the Luhansk and Donetsk regions following Russia's annexation of Crimea.
Mr Hollande and Mrs Merkel visited Kiev and Moscow earlier this week, in what appeared to be a speedily arranged visit to discuss the peace proposal.
Diplomatic talks have been continuing at an international security conference in the German city of Munich, where Russian Foreign Minister Sergei Lavrov said he "sincerely" hoped the latest peace plan would "produce results".
Mrs Merkel told the conference that while there was no guarantee diplomacy would succeed, it was essential to try. "I believe we owe that much to those who are affected in Ukraine," she said.
Mr Hollande told French TV that eastern Ukrainian regions would need extensive autonomy. "These people have gone to war," he said. "It will be difficult to make them share a common life."
The US is considering pleas to send weapons to Ukraine but Mrs Merkel said she could not "imagine any situation in which improved equipment for the Ukrainian army leads to President Putin being so impressed that he believes he will lose militarily".
The statement put her in opposition to Nato's top military commander, US Air Force general Philip Breedlove, who told reporters that Western allies should not "preclude out of hand the possibility of the military option".
Analysis: Jonathan Marcus, BBC News, Munich
The escalating winter war in Ukraine provides a grim backdrop to the talks in Munich. Behind the scenes here there have been serious efforts to try to breathe life into the peace process.
But in public nobody was pulling any punches. US Vice-President Joe Biden made clear Washington's distrust of the Russians and its determination to "allow Ukraine to defend itself".
Could that mean giving it weapons? That's the way US thinking seems to be going, to the horror of most of its European allies. The German Chancellor Angela Merkel for one spoke out passionately against such a move.
But there's really no new peace plan in the offing, only a revamp of the old one that was never implemented - the Minsk Agreement of last year. If diplomacy fails and President Barack Obama goes ahead with arms deliveries to Ukraine, it may not only divide Nato, but provoke an even more aggressive Russian response.
The BBC's Sarah Rainsford in Moscow says that any peace plan would have to address the route of any new ceasefire line - given the rebel advances of recent weeks - how to enforce it, and the future status of the conflict zone.
Russia is still denying any direct role in the conflict, while Kiev insists above all that Ukraine must remain united, our correspondent says.
In Munich, Mr Poroshenko brandished what he said were passports of Russian troops who had come to Ukraine.
Mr Putin, speaking at a labour union conference in Sochi, said there was "no war", but an attempt "to curb [Russia's] development". Western sanctions could not be effective against Russia, he said, but they could "cause certain damage".
Meanwhile, Nato Secretary General Jens Stoltenberg told the BBC that he had seen evidence of increased supply of heavy military weapons to the rebels, some "very advanced - they can only come from Russia".
Some 1.2 million Ukrainians have fled their homes since the Ukraine conflict began.
On Saturday, Ukraine's military said five servicemen had been killed and 26 wounded in the past day of fighting. At least seven civilians were reported to have been killed.
Ukraine also said rebels were amassing forces around the strategic town of Debaltseve and in Granitne, 35km north-east of Mariupol.
Michael Bociurkiw, a spokesman for the Organisation for Security and Co-operation in Europe (OSCE) told the BBC the situation was "getting very dire indeed".
More than 3,000 people had been evacuated in the past few days and some were being accommodated in cold conditions in train carriages, he said.