Germany and several other nations are meeting with the Turkish prime minister before the full summit to discuss a plan to resettle thousands of Syrian refugees directly from camps in Turkey.
Angela Merkel, the German chancellor, is leading the meeting of the so-called "coalition of the willing" involving 11 countries but the plan faces opposition from other European Union nations.
The full summit of 28 leaders will also debate a controversial plan for a new EU force that could shore up borders without the host country's consent, to stem a record flow of nearly one million migrants this year.
Cameron has vowed to "get a great deal for the British people" before holding a referendum on Britain's membership by the end of 2017, which could see it become the first country to leave the bloc.
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But the debate promises to be stormy as the other 27 leaders are almost unanimously opposed to Cameron's main demand -- a four-year wait before EU migrants working in Britain can claim welfare benefits.
"We want a fair deal with Britain but this fair deal with Britain has to be a fair deal for the other 27 too," Juncker told reporters ahead of the summit.
EU president Donald Tusk said there would be "no taboos", while officials from several EU countries said there appeared to be no concrete alternatives on the table to Cameron's benefits plan.
The debate in Britain has also been fuelled by concerns over the migration crisis -- the worst of its kind in Europe since World War II.
In many cases the root problem has been the same -- ideals of monetary and geographical union without the political architecture to back it up. But calls for "more Europe" fly in the face of an increasingly sceptical European electorate.