Migrants streaming across the Balkans reached Slovenia on Saturday, diverted overnight by the closure of Hungary’s border with Croatia in the latest demonstration of Europe's disjointed response to the flow of people reaching its borders.

Hungary’s right-wing government declared its southern frontier with Croatia off limits to migrants, blocking entry with a metal fence and razor wire just as it did a month ago on its border with Serbia.

Croatia began directing migrants west to Slovenia, which said some 300 had arrived and would be registered before continuing their journey to Austria and Germany, the preferred destination of the vast majority, many of them Syrians fleeing war.

But their movement had slowed visibly, with dozens of buses lined up at Serbia’s border with Croatia through the night and into Saturday as Croatian police controlled their entry, a Reuters reporter said. Slovenia suspended rail traffic with Croatia.

Aid agencies are concerned about backlogs of migrants building in the Balkans, battered by autumn winds and rain as temperatures drop before winter.

Hungary said it had reinstated border controls on its frontier with Slovenia, effectively suspending Europe’s Schengen system of passport-free travel. Both Slovenia and Hungary are part of the Schengen Area while Croatia is not.

A government spokesman said Budapest had taken the step because “migrants appeared” on the Slovenian side of the border.

Slovenia, a small country of two million people, says it can accommodate up to 8,000 migrants per day. Both Ljubljana and Zagreb say they will not restrict the flow so long as Austria and Germany keep their doors open.

Hungary says it is duty-bound to secure the borders of the European Union from mainly Muslim migrants threatening, it says, the prosperity, security and “Christian values” of Europe.

Budapest is among several ex-Communist members of the EU that oppose an EU plan to share out 120,000 refugees among its members.

That is only a small proportion of the 700,000 migrants expected to reach Europe’s shores by boat and dinghy from North Africa and Turkey this year, many of them fleeing war and poverty in the Middle East, Africa and Asia.

At a summit in Brussels on Thursday, the EU offered Turkey a possible three billion euros ($3.4 billion) in aid and the prospect of easier travel visas and "re-energised" talks on joining the bloc if it would help stem the flow of migrants across its territory.

But Hungary said this fell short of Budapest’s demands, which include formation of a common force to protect the borders of Greece, where most migrants arrive across the Aegean Sea from Turkey before heading north through Macedonia and Serbia.

Asked what would happen if Germany was to close its doors, Croatia's interior minister warned of a "domino effect".

“It will be a lot of trouble for all countries and I cannot predict what will happen in this situation,” Ranko Ostojic, speaking in English, told reporters at a migrant camp in the eastern Croatian village of Opatovac.

“They are risking their lives and nobody is able to stop this flow ... without shooting.”-Reuters