As the bloc's 28 leaders met in Brussels for a crucial discussion on their future strategy to deal with Moscow, EU President Donald Tusk said they should "keep all options open, including sanctions".
The leaders of France and Germany had given a similar sanctions warning after holding talks in Berlin with President Vladimir Putin on the eve of the Brussels summit.
A draft summit statement said the EU was weighing sanctions against supporters of Syrian President Bashar al-Assad's regime if they fail to stop atrocities, although it did not mention Russia by name.
"The EU should keep all options open, including sanctions if the crimes continue," Tusk said as he arrived for the summit.
British Prime Minister Theresa May, arriving for her first EU summit since the June Brexit vote, called for a united European approach to "sickening atrocities" in Syria,
"We must show a robust and united European stance in the face of Russian aggression," May told reporters. "It is vital that we work together to continue to put pressure on Russia to stop its appalling atrocities, its sickening atrocities in Syria."
The EU leaders will discuss their long-term Russia policy over dinner but deep cracks over how tough to be with Moscow remained evident.
Some warned there was no unity on adding sanctions over Syria to those that the EU has already imposed on Russia over the conflict in Ukraine.
"One option is sanctions. I don't think there's unity but I think it should be on the table, that this should be an option for the future," Swedish Prime Minister Stefan Lofven told reporters.
'Completely inhuman' – Russia had upstaged the summit by announcing earlier in the week that it would halt hostilities over Aleppo on Thursday just as the 28 leaders were gathering in Brussels.
It said Thursday it would extend the truce by 24 hours, without specifying when it would end. The UN had said earlier it had received a pledge from Moscow to extend it until Saturday.
But German Chancellor Angela Merkel said only a permanent ceasefire was acceptable.
"What is happening in Aleppo with Russian support is completely inhuman," she told reporters.
"That is why we have to work as fast as possible to get a ceasefire in place -- and not only one over several hours a day and than again many hours of bombardment, but a lasting ceasefire."
Moscow is Assad's strongest ally and came to his rescue last year when rebels appeared to be gaining ground.
"The EU is considering all options, including further restrictive measures targeting individuals and entities supporting the regime, should the current atrocities continue," the draft EU summit statement said.
"The European Council strongly condemns the attacks by the Syrian regime and its allies, notably Russia, on civilians in Aleppo," it added.
The EU is already embroiled in an angry standoff with Moscow over the conflict in Ukraine, complete with punishing economic sanctions on both sides.
Relations with Russia were put on the agenda of the two-day summit in Brussels months ago amid expectations of progress on Ukraine.
Since then, the deepening of the Syrian crisis has poisoned the atmosphere while the Berlin talks with Putin on Wednesday produced no real progress.
After the Berlin meeting, Merkel and French President Francois Hollande refused to rule out sanctions.
"Everything that can constitute a threat can be useful," Hollande said at a press conference, while Merkel added that "we cannot remove this option".
European Council President Donald Tusk arrives for an European Union leaders summit on October 20, 2016 at the European Council, in Brussels
European Union leaders warned Russia at a summit on Thursday that they will consider sanctions over its role in the Syrian conflict if Moscow does not stop "crimes" in the devastated city of Aleppo.
