A migrant pulls a boy inside a train through a window at the Keleti train station in Budapest, Hungary, September 3, 2015.[Photo/Agencies]

European laws, known as the "Dublin rules", require asylum seekers to apply in the country where they enter the EU and stay there until their applications are processed, even though most members of the 28-nation bloc have no border controls between them.

But countries like Italy, Greece and Hungary - where most first enter the European Union - say they have no capacity to process applications on such a scale and Austria has just let migrants travel onwards.

Interior Minister Johanna Mikl-Leitner said the situation in Hungary and measures taken by Budapest showed that countries in Europe urgently needed to find a coordinated approach.

"If there is no pan-European solution, then more and more member states will come up with measures to curb the flow of migrants, and that can't be the solution," Mikl-Leitner told journalists.

Volunteers were waiting in Vienna's main train stations to supply arriving migrants with food and water, as they did earlier this week, while medical centres have also been set up.