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A boy waited for his family to disembark from the boat that took them to Lesbos, Greece, on Friday. Lesbos has been the main entry point for Middle Eastern migrants to Europe.Credit Sergey Ponomarev for The New York Times

WARSAW — Hungary closed its border with Croatia early Saturday to stem the flow of thousands of migrants who had been making their way into the heart of Europe — just over a day after European Union leaders failed to reach a deal to seal the bloc’s outer borders.

The Hungarian foreign minister, Peter Szijjarto, told reporters in Budapest on Friday that his country was taking the action for two reasons: it had finally completed a razor-wire fence along the 216-mile border and, more crucially, because the European Union had yet to come up with a comprehensive plan to deal with the flow of migrants.For the last month, a migrant trail has operated freely, with countries along the route from the Balkans to Germany allowing those seeking asylum to pass through their territories — as long as they either applied for asylum or exited on the other end.The arrangement has helped to avert the kind of humanitarian crises that erupted in Central Europe in late August and early September, when migrants fleeing war and poverty in the Middle East, Africa and elsewhere began pouring into the Balkans in huge numbers.Continue reading the main story Closing the Back Door to Europe

In recent months European nations have worked to block the main route taken by migrants fleeing war and upheaval.

Hungary’s unilateral move threatened to upend the situation, but other countries in the region quickly said they were prepared to take up the slack — for the time being.

Officials in Croatia, which had been moving thousands of migrants into Hungary since that country had closed its border with Serbia a month ago, said they simply would shift the human flow west, into Slovenia, which had been preparing for such an eventuality. Croatia and Slovenia insisted, though, that they would be forced to close their borders, too, if Austria and Germany decided to stop accepting the migrants.

Slovenian officials said that while they could not accept an endless human surge, they had built the capacity to move as many as 6,000 migrants a day into Austria, and would continue to do so as long as the path remained open.

Speaking after a meeting of the Hungarian national security committee on Friday afternoon, Mr. Szijjarto said the government had alerted Croatia, as well as Austria, Germany, Poland, Slovakia and Slovenia, about its decision to close the border.

“The important thing is we expected this to happen and we are prepared for it,” said Slovenia’s foreign minister, Karl Erjavec. “Our goal is to assure security and order at our national borders and at the E.U. border and, at the same time, offer migrants a proper, humane treatment.”

Croatian officials, though, worried that Hungary’s move would cause a “chain reaction” of neighboring countries also blocking migrants. “If Austria closes its border, then Croatia must close its border, as well,” said Croatia’s foreign minister, Vesna Pusic.

Hungary approved the plan to close the crossing after European Union leaders agreed on Thursday to improve border controls and backed a deal with Turkey intended to slow the influx of migrants into Europe. But the deal with Turkey was not made final, and Chancellor Angela Merkel of Germany is headed to Istanbul this weekend to meet with President Recep Tayyip Erdogan to try to reach an agreement.

Aid groups and refugee officials have warned for months that the closing of any borders between Greece, where many of the migrants first enter Europe, and their intended destinations in Germany, Sweden or elsewhere will quickly cause a backup at borders along the so-called West Balkan route. That would lead to new encampments of stranded migrants behind blocked borders.

Hungary closed its nearly 109-mile border with Serbia last month after more than 350,000 migrants entered the country this year through the West Balkans, en route to Germany and other destinations.

As a result, the migrant flow shifted west, into Croatia, and until now people had been allowed to register and cross the Hungarian border on their way to Austria and beyond.

Slovenia has built 12 reception centers, with up to 7,500 beds. With winter approaching, the country is also scrambling to find sufficient heated accommodations for the migrants should they be delayed on their journey to Austria.

For months, Hungarian officials had complained that the failure of Greece, the migrants’ initial entry point, to control the human flow has shifted the burden to Hungary and other European Union nations.

Graphic Seeking a Fair Distribution of Migrants in Europe German and European Union leaders have called for European countries to share the burden of absorbing the hundreds of thousands of migrants who have poured into the continent this summer.