Walling Greece off will not make the European migration crisis go away, officials warned Tuesday, two days ahead of a European Union summit that could further expose rifts within the bloc widened by calls for stricter border controls.

In Brussels, European Commission President Jean-Claude Juncker joined the criticism of calls for border fences, warning that it could lead to the renewed fragmentation of Europe.

"I am not a supporter of the closing down of borders in Europe. I find it aberrant that, because of a lack of sufficient controls at our external borders ... we resign ourselves to again become provinces," he said.

"Either we are a continent or we are a grouping that subdivides itself into national categories. It is the wrong path that some member states are in the process of taking," he told national parliamentarians.

In Athens, European Council President Donald Tusk warned that removing Greece from the EU passport-free Schengen zone because it is the gateway for migrants striving to reach Europe's wealthy core will not solve any problems.

Several central European EU members on Monday accused Greece of doing nothing to stop an influx of migrants who then move on to other parts of Europe. Those countries have called for an effective blocking of the borders of the southernmost Balkan country.

"Excluding Greece from Schengen solves none of our problems. It does not end the war in Syria. It does not end Europe's attraction of migrants," Tusk told reporters after meeting Greek Prime Minister Alexis Tsipras.

In Tusk's words, the migration crisis "is testing our union to its limits." "We must protect EU borders together," he said. 

He is touring European capitals ahead of the EU summit, in an attempt to drum up support for EU unity as a gap widens within the bloc due the migration crisis. That unity is further threatened because Britain is seeking controversial reforms ahead of a referendum on membership.

Tsipras said that never before had the EU seen so many grave challenges and appealed for unity. "It is not a time for exits, but for joint action," he said. The Greek leader urged the EU to meet its commitments, including in the relocation of refugees.

Earlier, Greek Defence Minister Panos Kamenos said that the country has completed four of five hotspots - major centres for reception and registration of migrants that are a key ingredient of the EU plan for processing asylum seekers.

Along with the first operational hotspot on Lesbos, centres are now also operational on Leros, Chios und Samos, Kamenos said.

The final planned hotspot, on Kos, is due to start working within a few days, he said.

Criticized for delaying the hotspots and other facilities, Greece has now also completed a relocation centre on the mainland, at Schisto, with space for 4,000, while another one, at Thessaloniki, is due to be completed next month.

The implementation of agreed steps to fix the migration crisis, including control of the flow via Greece to northern Europe, is expected to be one of the main focal points of EU leaders this week. The other is agreeing on the British reforms deal.

The leaders are expected to call for an "end to the 'wave-through approach'" that has allowed migrants and asylum seekers to travel largely unhindered from Greece through the Western Balkans and on to wealthy countries such as Germany, according to a draft statement seen by dpa.

They are also expected to demand "further, decisive efforts" from Turkey, which has been the departure point for those traveling to Greece.

Under a deal struck with Ankara, Brussels is helping to pay for the upkeep of Syrians in Turkey, in return for greater efforts to stem migration flows and hamper smuggling networks.

Regardless of all plans to control the surge - which has seen around 1 million migrants reach Western Europe after crossing Greece, Macedonia, Serbia, Hungary or Croatia and Slovenia - Austria on Tuesday said it is putting additional measures in place for a quick sealing of its borders.

Saying that lacking coherent European action left it no choice, Austria - the first of the wealthy countries the migrants reach on their journey - worries that the present ebb in arrivals will turn to a new wave when weather in the Aegean improves.

"We will therefore strengthen border control efforts in a massive way," Chancellor Werner Faymann told reporters Tuesday in Vienna.

Interior Minister Mikl-Leitner was scheduled to provide details on the preparations for extended controls later Tuesday.

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