European leaders hoping to close out 2017 with a display of unity at their year-end summit instead found themselves scrambling to paper over a dispute on migration that exposes the Continent’s widening East-West divide.

The clash, triggered earlier this week after European Council President Donald Tusk called the EU’s attempts to impose mandatory refugee quotas “highly divisive” and “ineffective,” overshadowed the opening of a two-day summit that had been expected to revolve around Brexit and eurozone reform.

In the end, the leaders made little headway on migration and diplomats close to the talks said agreement on a new policy is unlikely before mid-2018, after Italy’s general election.

The sudden reemergence of migration at the top of the agenda offers a stark reminder of how little progress the EU has made over the past two years in resolving the highly emotional political issue, one that for leaders on both sides of the divide cuts to the core of what it means to be European. The tension also reflects the divergent political agendas of Europe’s eastern and western halves, underscoring the difficulty the EU will face in the coming months as its key leaders seek to build consensus for an ambitious overhaul of the bloc.

“These divisions are complicated by emotions, which make it hard to find even common ground and rational argument for this debate,” Tusk said at the opening of the summit. “This is why we should work on our unity more intensively and effectively than before.”

A number of leaders accused Tusk of undermining that aim by declaring in his letter to capitals ahead of the summit that the EU has “neither the capacity nor the legal responsibilities” to cope with the refugee crisis. Instead, he called for “decisive action by the lead member states,” a nod to Germany and Italy, and emphasized the need for strict controls on the EU’s external border. Tusk's remarks come as several Central European countries face possible sanctions for not accepting refugees under the EU's relocation program.

“We not only need solidarity in terms of regulating and controlling migration at the exterior borders — that is good, that is important — but we also need solidarity in terms of interior issues,” German Chancellor Angela Merkel shot back on her way into the meeting. “Selective solidarity, in my opinion, cannot exist among EU member states.”

Merkel has been trying ever since the outbreak of the refugee crisis in 2015 to convince other EU countries to share the burden that Germany and a handful of other members have taken on. Most countries, especially those in Central and Eastern Europe, have refused, citing everything from worries about terror to concerns over cultural identity.

The German leader made no secret of her annoyance over Tusk’s attempt to reframe the migration debate by effectively removing the pressure on Eastern countries to take in refugees, according to people in the meetings.

The episode highlights the difficulty Berlin faces in fighting for its priorities in Brussels under a caretaker government. Merkel is known for quietly dominating EU summits, but without a clear mandate, her influence is blunted.

“There is no human right to travel to the European Union and the European Union must protect itself” — Slovakia’s Prime Minister Robert Fico

Tusk’s offensive also appeared aimed at signaling to Central and Eastern European countries that their concerns are taken seriously by the EU’s leadership.

He has challenged Merkel before, most notably over the closure of the so-called Western Balkan route, once the preferred migration path for refugees to Northern Europe. While Merkel opposed the move, arguing it would strand refugees in Greece, Tusk argued the plan had worked, throwing his weight behind the Austrian-led effort in early 2016.

More generally, political leaders across the eastern region have complained they are treated like second-class members. That impression was reinforced last month when Central Europe lost out in the race to win one of the EU agencies relocating from the U.K. to the Continent as a result of Brexit.

There’s also growing concern that proposed initiatives to create a “two-speed Europe,” with a deeply integrated core around France and Germany, would leave Central and Eastern European countries in the cold.

Recurring threats from Brussels to sanction Poland and Hungary for allegedly undermining democratic principles and the rule of law, meanwhile, have led to accusations that the EU is trying to infringe on the countries’ hard-won sovereignty.

Tusk, a former Polish prime minister, understands those sensibilities better than most, though he is often accused by populist politicians at home of trying to undermine Warsaw.

On Thursday, even Poland’s ruling Law and Justice party, which often describes him as an enemy, appeared pleased with the Council president’s refugee stance. Tusk’s position hews closely to a series of proposals put forth by the so-called Visegrad Group of Central European countries to offer financial and operational assistance to confront the refugee crisis in lieu of taking in those fleeing.

“We want to help people affected by war in their own countries,” Mateusz Morawiecki, Poland’s new prime minister, said. "There this help is more effective.”

Others leaders from the region were more blunt.

“There is no human right to travel to the European Union and the European Union must protect itself,” Slovakia’s Prime Minister Robert Fico said.

Such talk raises the hackles of Western European leaders, who accused their Eastern counterparts of shirking their responsibility. Dutch Premier Mark Rutte called the Visegrad proposals “shameless.”

European Commission President Jean-Claude Juncker, who has been criticized in the past for giving the region short shrift, has engaged directly with the Visegrad countries in recent months and met with them ahead of this week's summit. “Sometimes it seems there are misunderstandings, that we don't know what the other thinks,” he said.

Despite the ongoing tensions, he insisted the atmosphere had improved and the region was “fully aligned” with much of the rest of Europe on migration.

Jacopo Barigazzi contributed reporting.