Leaders of four Central European states met in Prague on Monday with controversial plans to assist Macedonia in sealing its border with Greece, as some EU members look outside the bloc for ways to clamp down on migration flows.

The moves are a direct challenge to German Chancellor Angela Merkel ahead of an EU summit this week. European countries have been struggling to get to grips with a migration surge that saw more than 1 million people reach the continent last year.

The crisis has strained relations within the EU and triggered the return of border controls two decades after they were lifted.

With Greece coming under fire for allowing people to enter and cross its territory unchecked, attention has shifted to its border with Macedonia, a Balkan country outside of the EU that is next in line on the so-called Western Balkan migration route to northern Europe.

The Visegrad Group, comprising the Czech Republic, Hungary, Slovakia and Poland, has been sharply critical of Greece over its inability to prevent thousands of migrants from arriving via the Aegean Sea from Turkey.

"There is the possibility to stop illegal economic migrants on the borders of Macedonia and Bulgaria," Czech Prime Minister Bohuslav Sobotka said ahead of the conference.

The invitation of representatives from Macedonia and EU member Bulgaria to the meeting was "a sign of solidarity" with the two countries, which "must not be left alone in the crisis," Sobotka said.

Slovakia and the Czech Republic have sent a contingent of police officers to patrol Macedonia's border with Greece, and Polish Foreign Minister Witold Waszczykowski said Sunday that Warsaw could soon send more.

But it was not immediately clear what measures the Visegrad four are planning.

Macedonia is already expanding and reinforcing a fence along the border with Greece, on the main corridor of the migration flow arriving from Turkey, to hamper trespassers.   

Closing off the frontier would put further pressure on Athens, which fears that closing the Balkan borders would strand tens of thousands of migrants in the European Union's south-easternmost country.

Greece has been told it has to improve controls along its sea border or else face the possibility of other countries in the Schengen passport-free travel zone reintroducing controls on their borders for up to two years.

That would effectively amount to excluding Greece from Schengen, many have argued. The Schengen zone was launched in 1995 and now comprises 26 European countries.

The idea upset Merkel, who expressed her concerns to the German daily Stuttgarter Zeitung.

"To simply build a fence in Macedonia, which is not even an EU member, without a thought about what it would do to Greece, it is not just an un-European act, but it would also not resolve our problems," Merkel said in comments for Tuesday's edition of the paper.

The German ambassador to Prague, Arndt Freytag, urged "constructive solutions" in an interview with the daily Pravo and warned against the building of fences in Europe.

"The atmosphere in Europe is turning increasingly poisonous," he said, pointing to a growing split between western and eastern European countries.

But Visegrad leaders remained defiant.

Slovakian Prime Minister Rober Fico said that Germany made a mistake by welcoming migrants last year and was now trying to pressure others to deal with the consequences.

The nationalist Hungarian Prime Minister Viktor Orban, who sealed his country's border to migrants coming through Serbia and Croatia by erecting a fence, promised to further strengthen and expand the border protection and fight the redistribution of migrants within the EU.

In Brussels, meanwhile, the European Commission warned against sealing off Greece.

"The European response to the refugee crisis will be done with Greece, not against Greece," commission spokesman Margaritis Schinas added.

The EU's executive announced Monday that it will provide Macedonia with another 10 million euros (11 million dollars) to help the country improve its border and migration management systems.

But Tove Ernst, a spokeswoman for EU Migration Commissioner Dimitris Avramopoulos, noted that this aid "will not help to finance the construction of any fence."

"The commission's work is aimed at managing borders and not closing them," she added.

Another 12.7 million euros will be funnelled to Greece as emergency funding for the installation of prefabricated houses to host migrants and refugees on its mainland, the commission said.

Struggling to deal with the crisis, the EU overcame the hostility of the Visegrad Group countries and imposed a plan to redistribute asylum seekers across the 28 member states, but Hungary and Slovakia legally challenged the programme.

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