"What is happening in Aleppo is a war crime, one of the first demands is that the bombardments by the regime and its (Russian) backers must end," French President Francois Hollande said after a meeting between the three leaders in Berlin.
German Chancellor Angela Merkel condemned the air raids on Syria's second city as "inhumane and cruel".
Both leaders warned that they could not exclude imposing sanctions on Russia, hours ahead of an EU summit where Russia's role in Syria is set to be discussed.
"Everything that can constitute a threat can be useful," Hollande said at a press conference, while Merkel added that "we cannot remove this option".
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Turning to a planned ceasefire due to begin in Aleppo later this morning, Hollande said that Putin appeared to be ready to extend the truce, set to last for 11 hours.
"We came out of the meeting with the impression that there could be an extension of the truce, but it's up to the Syrian regime and Russia to show it," he said.
A truce of just a few hours would not be enough to deliver the necessary humanitarian aid and allow civilians to leave the area, Hollande added.
The Syrian army separately said that the ceasefire would last three days.
Aleppo, held by rebels determined to oust President Bashar al-Assad, has come under heavy bombardment since the Russian-backed military announced an offensive in late September to regain control of the east.
Air strikes there have flattened numerous residential buildings and civilian facilities, in a campaign the European Union said could amount to war crimes.