VIENNA - Austria said Wednesday it would build a fence along its border with fellow EU state Slovenia to “control” the migrant influx, in a blow to the EU’s cherished passport-free Schengen zone.

Both countries have become key transit points for tens of thousands of refugees and migrants seeking to reach northern Europe ahead of the winter, and before more potential EU border closures.

Vienna’s announcement prompted sharp criticism from Berlin amid an escalating spat over Austria’s approach, underlining how the crisis is straining relations within the EU.

“We do not believe that the current migrant crisis that Europe is facing can be resolved with the building of fences or walls,” German government spokesman Steffen Seibert said.

The small Alpine nation of Slovenia also reiterated its readiness to erect a fence along its Croatian frontier if new EU plans aimed at improving the situation failed to produce quick results.

Austria’s move will intensify concerns about the EU’s cherished Schengen system, a crucial part of European integration efforts, which allows for the free movement of people and goods. But Interior Minister Johanna Mikl-Leitner insisted the barrier was “not about shutting down the border”.

“This is about ensuring an orderly, controlled entry into our country. Also, a fence has a gate,” she said Wednesday.

Big rifts have opened up between EU members over how to handle Europe’s worst refugee crisis since World War II, which has seen more than 700,000 people land on the continent’s southern shores so far this year, the majority from Syria, Iraq and Afghanistan.

The flow has overwhelmed nations along the migrant trail from Greece through the Balkans, already prompting Hungary - an EU and Schengen member - to seal its southern borders with razor wire. The human cost of the crisis rose on Wednesday as Greek officials found the body of a seven-year-old boy near the island of Lesbos, with six more migrants also feared drowned.

More than 3,200 are estimated to have died in the Mediterranean since January. European Commission President Jean-Claude Juncker met Austrian Chancellor Werner Faymann to discuss the situation, as the EU said it had “not been notified” of Vienna’s decision.

Just days ago Juncker and other EU leaders at an emergency Balkans summit warned that “unilateral actions could trigger a chain reaction”.

Meanwhile, rescuers plucked more than a thousand migrants from overcrowded boats near Libya on Wednesday morning, Italy’s coast guard said, the most reported saved in a single day in three weeks. The flow of migrants crossing the Mediterranean in rickety boats from Libya towards Italy’s southern tip had been abating with deteriorating weather conditions and as more people chose alternative routes into Europe.

About 140,000 have reached Italy this year in boats from Africa out of a total of about 700,000 who have come to Europe by sea, mostly through Greece.

British, Irish, Slovenian and German navy ships took part in Wednesday’s rescues along with the Italian coast guard and a ship operating under the auspices of European Union border agency Frontex.

Italy and Greece have struggled to cope with arrivals as civil war in Syria and deprivation and human rights abuses in Africa has driven the biggest wave of migration towards Europe since World War Two.

Few details have been released so far about the planned barrier, which is set to run several kilometres either side of the Spielfeld border crossing, where thousands of migrants have arrived in recent weeks.

Mikl-Leitner - who last week said it was time to build “fortress Europe” - stressed Wednesday the situation could escalate as winter sets in and migrants grow impatient at having to wait at border posts in cold weather. She pointed the finger at Germany, which is expecting up to one million asylum-seekers this year, saying border police there processed too few migrants and criticising Chancellor Angela Merkel’s open-door policy.

Her German counterpart Thomas de Maiziere hit back, accusing Austrian border police of waving through thousands of migrants without properly informing local authorities.

At the Austrian-German border, diplomatic spats were low on the migrants’ list of worries. “In Syria, I left because of the bombs and now, here, it’s like a prison. They tell us nothing, we know nothing,” said one man named Walid.

“Yesterday, some people of my family could go but not me and my two daughters. The German police didn’t explain why, and now my family is cut in two.”

Germany is seeking to crack down on economic migrants, with tens of thousands of rejected asylum applicants from Balkan countries due to be sent home in the coming months.

De Maiziere has also urged Afghans to stay and rebuild their wartorn country instead of coming to Europe.

Further down the migrant trail, Slovenia has been struggling to cope with the influx - nearly 90,000 have arrived in the last two weeks, since Hungary sealed its frontier with Croatia.

Ljubljana warned it too would shut its border if the EU did not stick to a 17-point action plan announced at Sunday’s Balkans summit.

“As a European I do not desire (a fence) but the state will be forced into it if the commitments are not fulfilled,” Prime Minister Miro Cerar said Wednesday.

As part of the plan, the EU has pledged to send 400 police officers from other bloc members to Slovenia. It has also vowed to set up 100,000 places in reception centres in Greece and the Balkans.

The crisis has boosted the far-right across Europe, including in the Netherlands where Geert Wilders’ anti-immigration Freedom Party now leads opinion polls.

Dutch King Willem-Alexander and senior politicians on Wednesday called for calm as tensions flared over sheltering around 60,000 asylum-seekers this year.