The shadows of Russia and Brexit loom over a "

The EU leaders will hold dinner without British Prime Minister Theresa May where they will try to present a united front over how they handle Britain's departure from the bloc.

They are also expected to rollover sanctions against Moscow over the conflict in Ukraine for another six months, but are unlikely to impose any new measures in response to the carnage in Aleppo.

They will further hold crisis talks about a pact with Ukraine that the Netherlands has threatened to torpedo if the EU refuses to offer guarantees against further integration for Kiev.

"We are treading on a minefield, there are so many issues on the agenda that still can go wrong," warned a senior EU European Union official.

The one-day summit, cut back from the usual two days, wraps up an annus horribilis for the bloc that has seen it face a wave of populism including the shock Brexit referendum vote.

Dutch Ukraine worries

The thorniest issue on Thursday could be Netherlands Prime Minister Mark Rutte's efforts to negotiate a way past a referendum in April in which his country voted against a key EU-Ukraine pact.

The Netherlands is the only one of the 28 EU states that has not ratified the deal so far, and Rutte wants vows that the pact is not a first step towards EU membership for Ukraine, and that it will not provide defence guarantees to Kiev.

Agreement will for once be far easier on the EU's stiff economic sanctions against Russia over the war in Ukraine, which were imposed in 2014 after the shooting down of Malaysia Airlines flight MH17.

German Chancellor Angela Merkel and French President Francois Hollande will debrief leaders after which "I expect that there will be a consensus" on a six-month rollover, a German government official said.

The rollover is expected despite little appetite among some countries, and fears that US President-elect Donald Trump is set to take take a much softer line on Moscow amid signs of rapprochement with Vladimir Putin.

The EU leaders are however set to stop short of threatening any new sanctions against Russia over the violence in Syria's Aleppo, where the army has seized nearly all the ravaged city.

The leaders will say in a statement obtained by AFP that the EU "strongly condemns" the assault by the "Syrian regime and its allies, notably Russia" and calls on them to allow aid in to Aleppo.

But they will go little further than they did at their last summit in October when they were equally divided, saying only in a draft statement obtained by AFP that "the EU is considering all available options."

No dinner for UK PM

Britain's May will then leave the summit while the other 27 discuss the process for dealing with Britain's departure once May triggers the two-year divorce process, which she has promised to do by the end of March 2017.

They will issue a statement saying they agree to launch talks as quickly as possible after she does so.

The 27 will also agree that the European Commission will lead the talks, under French chief Brexit negotiator Michel Barnier, but that the European Council of member states must also be involved in the talks.

But that has prompted an angry response from European Parliament chief Martin Schulz, who warned that MEPs could reject any deal with "grave consequences" unless they are also included.

At the moment MEPs must approve any deal but will not be represented in the talks.

The EU 27 have refused all negotiations with Britain until the process begins, while warning that May is likely to have only a short window until October 2018 to broker a deal.

Brexit minister David Davis said Wednesday a transitional deal to cover the period after the two years before a full trade agreement is reached was possible "if necessary".