Road and rail routes to northern Europe from the Balkans were closed to migrants yesterday leaving thousands stranded in countries on the doorstep of Europe.

Overwhelmed Croatia began transporting thousands of migrants to its border with Hungary, admitting it cannot cope with the flood of humanity seeking a route through to northern Europe.

The move came as Budapest started building a new fence to keep them out, potentially setting up a new flashpoint in Europe's worst migration crisis since World War II.

With no let-up in the flow of people desperate to find shelter in Europe, new figures showed the European Union had received almost a quarter of a million asylum requests in the three months to June.

A record 473,887 refugees and migrants have crossed the Mediterranean to Europe so far this year, the International Organization for Migration said, most of them from countries at war such as Syria who are seeking a better, safer life.

With Europe's cherished borderless travel zone in tatters, the continent has been wrangling over how to handle the influx, with eastern states ruling out an EU quota plan for sharing out the burden.

After EU frontline state Hungary sealed off its southern border with Serbia earlier this week, thousands of migrants and refugees have now headed westwards, seeking to open up a new route westwards through Croatia and Slovenia into Austria.

But after two days in which they registered more than 13,000 Croatia yesterday said it had reached saturation point, closing seven of the eight crossings along its eastern border with Serbia.

"Our capacities to take in more are saturated," Interior Minister Ranko Ostojic told N1 television station.

As the crowds began making their way westwards, Slovenia announced it was suspending all Croatian rail links until late yesterday.

As the chaos spread, the UN's refugee agency (UNHCR) and the International Committee of the Red Cross issued separate pleas for the 28-member bloc to agree on a common policy for handling the situation and minimising the human misery.

"The countries affected must work together closely instead of closing their borders like they are doing at the moment," ICRC President Peter Maurer told German weekly Der Spiegel.

And UNHCR spokesman Adrian Edwards warned that time was running out, saying recent events in Hungary and elsewhere "demonstrated the chaos and confusion being caused by the absence of a coherent and united response to Europe's refugee situation."

Two key EU meetings due to take place next week are "crucially important" for reaching some form of agreement, he said.

"These occasions may be the last opportunity for a positive, united and coherent European response to this crisis. Time is running out."

German Foreign Minister Frank-Walter Steinmeier yesterday said that EU members reluctant to accept migrant quotas may have to be outvoted and overruled in the 28-member bloc.

"It just cannot be that Germany, Austria, Sweden and Italy carry the burden alone," he said. "That's not how European solidarity works.

As Germany has opened its gates, Hungary has done the opposite, sparking a wave of anger as it sealed off its southern border with Serbia and announcing plans for a similar fence along its frontier with Romania.

And on Friday, its hawkish premier Viktor Orban said hundreds of troops had begun work on a 41-kilometre (25.5-mile) section of the 330-kilometre border along its frontier with Croatia to keep migrants out.

"There will be no sandhill or molehill to hide behind, we will defend our borders," he told Hungarian public radio.

However, hundreds of migrants crossed into Hungary from Croatia yesterday, handed over by bus under the watch of Hungarian security forces sworn to keep them out.

Police and soldiers rushed to take position as a convoy of more than 10 Croatian buses offloaded hundreds of migrants who had crossed in the past two days into EU-member state Croatia from Serbia after Hungary blocked their path into the European Union with a metal fence on its border with Serbia.

Soldiers maneuvered two Humvees mounted with machine guns to block their route. But minutes later, the migrants were directed to board Hungarian buses bound for an unknown destination. A police statement said they would be registered, without giving details.