Brussels - A bitter divide on how to tackle migration. Differing visions on the future of the eurozone. Efforts to minimize the pain of Brexit. These are just some of the tough issues awaiting EU leaders set to gather in Brussels on June 28-29.

Europe united despite divisions

The summit comes at a time when the EU is at pains to show unity and resolve on the world stage, especially as US President Donald Trump is tearing up the international rulebook and testing trans-Atlantic relations.

"The Atlantic has become wider under President Trump and his policy of isolationism has left a giant vacuum around the world," German Foreign Minister Heiko Maas said in a recent speech.

"Our common response to "America First" today must be "Europe United," he added.

But the EU is grappling with deep divisions, with none so pressing as the issue of migration. An influx of more than 1 million people in 2015 exposed serious flaws in the EU's asylum rules, overwhelming capitals and triggering an anti-immigrant backlash.

Despite a sharp reduction in the number of arrivals since then, the issue is back at the top of the agenda.

Last week, Italy's new eurosceptic government - elected on an anti-immigrant platform - triggered outrage when it closed its ports to a rescue ship carrying 630 migrants, demanding more help from the EU.

In Germany, meanwhile, Chancellor Angela Merkel's shaky government coalition is teetering due to a challenge from within her own ranks, demanding that migrants who had already registered for asylum elsewhere in the EU, such as Italy, be kept out.

Reform of EU's asylum rules

Merkel has been given two weeks to reach deals with other EU states or else be overruled by Interior Minister Horst Seehofer who - driven by a fear of far-right gains in upcoming Bavarian regional elections - has threatened to selectively close the border to such migrants.

The EU summit was originally supposed to deliver a breakthrough in long-running efforts to reform the EU's asylum rules, which have stalled amid the steadfast opposition of countries including Hungary and Poland to the idea of relocating refugees from front-line states.

Instead, EU leaders are now looking for new ideas they can rally around, such as better efforts to secure the bloc's external borders or even migrant reception centres outside the EU.

Several EU leaders, including Merkel and Italian Prime Minister Giuseppe Conte, are due to tackle some of these issues at emergency migration talks convened ahead of the EU summit on Sunday. Whether this patches up differences or exposes them remains to be seen.

Besides migration, EU officials had hoped for sign-off next week on a road map for eurozone reforms, following an initiative by Merkel and French President Emmanuel Macron to bolster the 19-member currency bloc against future shocks.

The Franco-German proposals, agreed earlier this week, include a fledgling eurozone budget and plans to turn the eurozone's bailout fund into a European Monetary Fund better equipped to support stricken economies.

But it is far from certain that other member states will get behind the proposals - a point European Commission President Jean-Claude Juncker conceded after attending the Franco-German summit in Meseberg, north of the German capital.

Future of relations with Britain

Opinions on the eurozone budget "diverge" amid uncertainty about how it would be implemented, several of the currency area's finance ministers said following lengthy discussions on Thursday.

Dutch Finance Minister Wopke Hoekstra was blunter, asking "what type of problem" the eurozone budget would solve. "That is unclear to us, and we're not in favour of it," he said ahead of the meeting with his eurozone counterparts.

Furthermore, many details have been left unclear, in what many see as a deliberate fudge to bridge differences between Macron's ambitious vision of reform and Merkel's more cautious approach, hemmed in by her conservative party.

One of the few summit issues on which EU leaders have been remarkably unified is the future of relations with Britain once it leaves the bloc next year.

The leaders of the 27 EU countries without Britain are expected to take stock of negotiations to secure London's exit terms, agree a transition period and outline future relations, all of which is supposed to happen by October.

"The hardest parts are still to do. And there is not much time left to find a concrete agreement," Juncker warned Thursday, in a speech to the Irish parliament.

The Irish border question is one of the thorniest, with Brussels warning that it could bring down the whole withdrawal deal.

"As the clock to Brexit ticks down, we must prepare for every eventuality, including no deal. This is neither a desired nor a likely outcome. But it is not an impossible one," Juncker added.