Divided EU leaders will seek a tougher strategy to deal with Russia at a summit in Brussels Thursday amid threats of sanctions over Moscow's involvement in Syria.

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.

“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.

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.

- 'Catastrophe' in Aleppo -

The split is mainly between eastern states looking nervously at the bear on their border and countries led by Italy and Greece that fear the loss of a key trade partner.

It was Italian premier Matteo Renzi who asked back in May for a debate on Russia that was not under time pressure from a sanctions decision, but that has now been largely overtaken by events.

The EU leaders are to issue a statement saying the bloc "strongly condemns the attacks by the Syrian regime and its allies, notably Russia, on civilians in Aleppo," and to call for an immediate halt to hostilities, according to a draft obtained by AFP.

"Sadly we could be looking at a quite different discussion had it not been for the catastrophe of the last few weeks and the Russian escalation," a senior European diplomat said.

But they will fall well short of endorsing new sanctions against Russia over Syria despite such calls from Washington, London and Berlin.

A German government official said: "This summit, as Tusk put it, will not take decisions, meaning that it will not take any options off the table."

The EU currently has wide-ranging economic sanctions in force against Russia over the Ukraine conflict, which were imposed after the shooting down of Malaysia Airlines flight MH17 over rebel-held eastern Ukraine in 2014.

The leaders are due to discuss their renewal for another six months at their next summit in December and there had been pressure earlier in the year for the bloc to scale them back or drop them.

Outrage over Russia's role in the "catastrophe" in Aleppo means "any instinct for dialling back the Ukraine sanctions has pretty much disappeared without trace," a senior EU diplomat said.

Meanwhile British premier May is set to make her eagerly awaited first appearance at a summit of a bloc that her country has voted to leave in a shock referendum result in June.

May will give a brief "information point" over dinner that would focus on her earlier announcement that she will trigger Britain's two-year divorce process from the EU by the end of March 2017, but there is not expected to be any major debate.

The EU leaders will also discuss aid deals with African nations to curb the biggest migration crisis since World War II.

Source: AFP