Athens escalated its dispute with Vienna over migration policy on Friday, rejecting a request from the Austrian interior minister to visit a refugee camp as it restricted transport from its migrant-crowded islands to relieve pressure on the mainland.

A Greek Foreign Ministry spokesman told dpa on Friday that Austrian Interior Minister Johanna Mikl-Leitner's wish to visit a refugee camp was turned down.

Vienna reacted coolly, with an Interior Ministry source telling dpa that "it is perfectly legitimate if Greek chose to hold this conversation at a later date."

The snub follows Athens' decision to recall its ambassador from Vienna, with Greece blaming the border restrictions imposed by Austria and Balkan countries for the backlog of refugees and migrants on its soil.

An estimated 24,000 people were stranded in Greece on Friday because its northern neighbour Macedonia and other countries on the Balkan route - Serbia, Croatia, Slovenia and, finally, Austria - have all progressively shut their borders in recent months.

In the latest limit on migrant transit, Serbian Interior Minister Nebojsa Stefanovic said Friday that Balkan route countries will allow only 500 refugees to transit per day.

Stefanovic told Belgrade's Pink TV that the new regime "is in effect as of today."

Since Sunday, only Syrians and Iraqis were allowed to transit, while Afghans, who were waved through in the previous three months, remained at Greece's border with Macedonia.

Authorities in Greece said 4,000 people were waiting at the border.

"Barriers do not solve any problems," European Commission President Jean-Claude Juncker said Friday after a meeting with Italian Prime Minister Matteo Renzi in Rome. "On the contrary, they make them worse for other European Union countries, especially in southern Europe and the Balkans."

With its northern border effectively shut and with migrants continuing to arrive from Turkey, Greece restricted the transport of people to the mainland to reduce pressure on its cities and at the border.

"We must slow the influx because of the backlog on the mainland," Shipping and Islands Minister Thodoris Dritsas told Skai TV.

Fewer ferry tickets will be sold to migrants, and fewer ferries will sail from the islands. People will "wait a little longer" before continuing their journey following the registration process, Dritsas said.

In order to avoid burdening the islands, the migrants will be sheltered on ferries docked in the ports of Lesbos, Chios and Samos, he added.

In Brussels meanwhile, the European Commission said it is preparing contingency plans to "avoid a difficult humanitarian situation," according to a spokesman for the European Union's executive.

EU President Donald Tusk announced plans Friday to visit the main countries along the Western Balkan route next week, in an effort to build "European consensus" on the handling of the migration crisis ahead of a special summit on March 7.

Migrants are still moving north through Greece, avoiding overcrowded refugee centres, in the hope that they will manage to get through and resume the journey towards wealthy EU countries, such as Germany.

Leaving tens of thousands of asylum seekers stranded could eventually lead to a humanitarian disaster, the Frankfurt-based humanitarian organization Pro-Asyl warned.

"We are warning of a situation in which there is risk of fatalities," the organization chief Guennter Burkhardt told dpa in Berlin. 

Burkhard said the EU's "isolationist policy" was driving people - most of whom hope for shelter - to turn to organized crime and smugglers for a run across sealed borders.

On Saturday, humanitarian organizations in Europe and North America are due to stage marches in 115 cities demanding open borders for refugees fleeing violence. The march, held under the motto Safe Passage Now, is supported on all major social networks.

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