While Russia and Syria began a brief "humanitarian pause" in the devastated city, the European Union is set to "strongly condemn" Moscow and call for a permanent end to hostilities.
The leaders of France and Germany warned after talks with Vladimir Putin on the eve of the summit that they could not rule out sanctions over the carnage.
Russian-Europe war rhetoric spikes
"Everything that can constitute a threat can be useful," President Francois Hollande said at a press conference following the meeting in Berlin, while Chancellor Angela Merkel added: "We cannot remove this option."
They did however say there had been progress on the Ukraine crisis that first plunged relations into the deep freeze two years ago, and over which the EU still has sanctions in force against the Kremlin.
Europe heads for Red Planet Mars in search of life
The 28 EU leaders will discuss their long-term Russia policy over dinner at the summit, but cracks over how tough to be on Russia remain evident.
Brexit will also rear its head with British Prime Minister Theresa May at the summit table for the first time, while the bloc's divisions will also be on display over the migration crisis and a troubled trade deal with Canada.
"Our main asset in dealing with Russia remains our unity. To date, regardless of our differences we have always remained united and we must continue to do so," EU President Donald Tusk said on Wednesday.
The timing of the Aleppo truce to coincide with the summit reflects how the Kremlin has long played on the divisions in Europe.