Angela Merkel and Francois Hollande are taking to Moscow a peace proposal crafted in the Ukrainian capital Kiev on February 5, but details have not been released.
Meanwhile a truce has allowed civilians to leave Debaltseve, at the heart of the latest fighting in eastern Ukraine.
Russia is accused of arming pro-Russian separatists – a claim it denies.
The Kremlin also rejects claims by Ukraine and the West that its regular troops are fighting alongside the rebels in the eastern Donetsk and Luhansk regions.
Clashes have left nearly 5,400 people dead since April 2014, the UN says.
A September ceasefire, signed in Minsk, Belarus, has failed to stop the violence. Since then the rebels have seized more ground, raising alarm in Kiev and among Ukraine’s backers.
Before he left for Moscow on February 6, Francois Hollande said the goal of his visit was not just a ceasefire, but a “comprehensive agreement” – though Angela Merkel said it was “totally open” whether that could be achieved.
Meanwhile Vice-President Joe Biden accused Russia of “continuing to escalate the conflict” and “ignoring every agreement”.
Joe Biden was speaking in Brussels, where he is meeting top EU officials.
He accused Vladimir Putin of continuing “to call for new peace plans as his tanks roll through the Ukrainian countryside”.
He said Russia could “not be allowed to redraw the map of Europe”.
Ukraine is also set to dominate an annual multi-lateral security conference in Munich.
The fighting has intensified in recent weeks after a rebel offensive, and a temporary truce was declared in Debaltseve on February 6, where Ukrainian forces are fighting to hold the town against surrounding rebels.
Convoys of buses travelled to the town on Friday to evacuate civilians who had been forced to shelter underground from the bombing.
They were escorted by monitors from the OSCE security watchdog, Reuters reported.
Washington is considering Ukrainian pleas for better weaponry to fend off the rebels, raising European fears of an escalation in the conflict and spurring the latest peace bid.
On February5, Francois Hollande and Angela Merkel examined the peace proposal with Ukrainian President Petro Poroshenko, without releasing any details.
Moscow says it is ready for “constructive dialogue” – though still denying any direct role in the conflict – while Kiev insists above all that Ukraine must remain united.
A spokesman for the Kremlin said President Vladimir Putin would discuss “the fastest possible end to the civil war in south-eastern Ukraine”.
Some 1.2 million Ukrainians have fled their homes since April 2014, when the rebels seized a big swathe of the Luhansk and Donetsk regions following Russia’s annexation of Crimea.
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