Britain's Prime Minister David Cameron speaks to the media as he arrives at the European Council in Brussels, on December 17, 2015
Brussels (AFP) - EU leaders warned Britain over its "unacceptable" reform demands and urged greater unity on the migrant crisis Thursday at a summit marking the end of one of the bloc's toughest ever years.
David Cameron was told by Francois Hollande and other top officials in the European Union that his calls for limits to benefits for EU workers in Britain threatened the 28-nation club's core principles.
The British premier vowed to "battle through the night" to make progress towards a deal at the next EU gathering in February, before holding a referendum on Britain's membership by the end of 2017.
But with Europe already deeply split by a year that has seen a record inflow of nearly one million mainly Syrian refugees, crises in Greece and Ukraine and terror attacks in Paris, his counterparts were in little mood for compromise.
"If it is legitimate to listen to the British prime minister, it is unacceptable to revise founding European commitments," French president Hollande told reporters as he arrived in Brussels.
European Council President Donald Tusk added that "some parts of the British proposal seem unacceptable," while European Commission President Jean-Claude Juncker urged Cameron to come up with alternatives.
"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 said.
- Cameron demands -
Cameron has broad support for his goals of greater protection of non-eurozone members, for an exemption from the EU's goal of "ever closer union" and for greater economic competitiveness.
But the rest of his counterparts, with the exception of Ireland and Denmark, oppose his demand for a four-year limit before EU migrants working in Britain can claim benefits such as social housing or child welfare payments.
Germany's powerful chancellor and potential deal-broker Angela Merkel said she wanted to avoid a so-called "Brexit", but would "not limit the fundamental principles of the EU."
Eastern European countries have benefited hugely from the ability to live and work elsewhere in the EU, especially in Britain, which says it's on track to overtake Germany as the union's most populous nation by 2050.
Poland, Hungary, the Czech Republic and Slovakia said they "will not support any solutions which would be discriminatory or limit free movement."
The summit -- the EU's 12th this year, a record -- is also dealing with the migrant crisis and the threat it poses to the Schengen area, the cherished European passport-free zone that symbolises that ideal of free movement.
Wide rifts have emerged after Merkel opened Germany's doors to Syrian refugees, causing huge strain on transit countries and prompting several to suspend the Schengen rules and reintroduce border checks.
The EU leaders are debating a plan for a new border and coastguard force that could intervene in member countries -- even without their consent -- in order to shore up frontiers and stem the tide of migrants.
- 'Painful' choices -
Yet many states are worried the plan means ceding sovereignty to Brussels, including Poland and Greece, the country that has seen by far the biggest number of migrant arrivals.
With Europe facing its biggest migrant crisis since World War II, Merkel said she "strongly supports" the border guard scheme, while Tusk -- a former Polish premier -- said any other solution would be "equally painful".
Merkel also hailed a "very good" meeting between 11 EU states and the Turkish prime minister on the sidelines of the main summit to discuss a plan to resettle thousands of Syrian war refugees directly from camps in Turkey.
But an EU report on Thursday said that a three-billion-euro ($3.2-billion) deal with Ankara in November to keep more refugees on Turkish soil had so far had little effect on the number of people crossing to Greece.
Other plans have been bogged down by divisions, with a deal for member states to take in 160,000 refugees from overburdened Greece and Italy resulting in just 208 people being relocated so far, largely due once again to opposition from eastern Europe.
EU leaders meanwhile look set to roll over sanctions against Russia over the Ukraine conflict, despite Italy insisting on delaying the decision from last week so that it could be discussed at the summit.