Europe's cherished system of borderless travel is increasingly at risk as countries grapple with record numbers of refugees and migrants clamouring to enter their territory. Those who do reach European Union countries quickly discover there are wildly differing rules for how asylum-seekers are treated.

German Chancellor Angela Merkel issued an unusually sober warning this week that the passport-free Schengen zone of 26 countries - hailed as a European success story - was under threat.

With Hungary sealing off its main railway station on Tuesday to stop asylum-seekers moving across the European Union, observers fear the genie is out of the bottle and other countries will restore internal borders to curb the flow of humanity.

"If Europe does not take steps as a whole, the current situation is worrying for free movement at the heart of the EU," Matthieu Tardis, a migration expert based in Paris, told AFP.

"Calling Schengen into question is a risk because each state deals with the issue in a very national way based on public opinion - and in the end what is called into question is the crossing of national borders."

Since its creation in 1995, the Schengen area - named after a border town in Luxembourg - has abolished passport controls for travel between 22 EU countries and non-EU Norway, Switzerland, Iceland and Liechtenstein.

Supporters say Schengen has been a boon for Europe's single market, slashing costs by doing away with armies of border bureaucrats and sweeping away psychological barriers among Europeans.

But the system is now creaking under the pressure of the biggest refugee crisis since World War II, as people from Syria, Afghanistan and Africa flood into Europe seeking refuge from war.