EU leaders will have their unity tested this week at a summit replete with political minefields, ending 2016 in much the same way as they started it.

A highly controversial bid by Britain to restrict the access of EU citizens to welfare benefits will be discussed, while the always thorny issue of sanctions against Russia is expected to elbow its way in. Migration, terrorism and the economy will round out the agenda.

"In times when geopolitics is back in Europe, we need to be united and strong," EU President Donald Tusk wrote in a letter to the 28 national leaders last week. "This is in our common interest and in the interest of each and every EU member state."

But economic crises and migration tensions have cracked the European Union's integrity. Fears are rife that Britain's upcoming referendum on EU membership could become a coup de grace.

British Prime Minister David Cameron has promised to hold the in-out referendum by 2017. To avert a so-called Brexit, he has proposed to cut a deal that would safeguard his country's interests.

Cameron is most controversially seeking to restrict access to British social benefits by requiring other EU citizens to work in the country for four years before qualifying, in response to British concerns that foreigners are saturating the job market.

"[This] will require a substantive political debate at our December meeting," Tusk wrote, saying there was no "consensus" among leaders on the issue so far.

London is nevertheless sticking by the demand.

"If people have other ideas that will deliver on this very important agenda for the British people, we're absolutely prepared to listen," British Foreign Secretary Philip Hammond told journalists in Brussels on Monday. "But at the moment, the only proposition on the table is our four-year proposal."

A breakthrough on the British demands is not expected at the summit on Thursday and Friday. Tusk expressed hope that a deal can be finalized in February, when EU leaders are next due to meet.

Before then, the bloc will have to make a decision on how to proceed with its economic sanctions against Russia, which were first instituted in July 2014 in reaction to its annexation of the Ukrainian peninsula of Crimea.

They are among the most controversial steps taken to influence Russia's course in Ukraine, having inflicted economic pain both on Moscow and the EU. The sanctions, which aim to hamper Russian imports and Russian banks' access to European capital markets, have also severely strained relations between Brussels and Moscow.

EU countries had reached an agreement in principle to prolong the sanctions for another six months because of halting progress on a ceasefire in eastern Ukraine. But Italy prevented a quick, low-level deal on the matter, which is now expected to be raised at the summit.

Diplomats say they do not think Italy intends to block the prolongation, but may try to establish a link with controversial plans by Russian energy giant Gazprom to expand its North Stream gas pipeline running beneath the Baltic Sea into Germany.

Italian Foreign Minister Paolo Gentiloni expressed confidence in Brussels on Monday that the EU will reach "a common decision" on the sanctions. A prolongation would require unanimity.

"Nobody is against ... sanctions, it's not a matter of principle," Gentiloni said.

"Sanctions against Russia must remain," Swedish Foreign Minister Margot Wallstrom told journalists in Brussels, according to the local Swedish representation. "EU unity in supporting Ukraine [is] key."

"I don't see major problems in any of the member states on the political decision," EU foreign policy chief Federica Mogherini added.

The economic sanctions are currently due to expire on January 31.

Also on the summit agenda are further actions that the EU could take to combat terrorism in the wake of the Paris attacks and to get a better handle on the waves of migrants flowing through Europe.

Member states willing to resettle asylum seekers from Turkey are expected to hold separate talks before the summit with that country's prime minister, Ahmet Davutoglu. But diplomats said on condition of anonymity that they do not yet expect an agreement on the matter.

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