Migrants cross the border from Serbia near Roszke, Hungary, September 14. Migrants sped through the Balkans by train, bus and taxi on Monday, racing to beat a border crackdown promised by Hungary's right-wing government. [Photo/Agencies]

BIGGEST THREAT TO SCHENGEN

Monday's measures were the biggest threat so far to the Schengen system of a border-free Europe, which ranks alongside the euro single currency as one of the transformative achievements of integration on the continent.

Named after a Luxembourg town where it was agreed, Schengen has eliminated frontier posts across the continent since 1995. Twenty-six European countries now issue common visas and leave the borders between them unguarded.

Frontiers which were fought over for centuries and which were a bottleneck for traffic and trade just a few years ago are now marked by little more than signposts on highways across the world's biggest economic bloc.

The rules bar undocumented migrants from travel within the zone but leave few mechanisms to stop them.

That has created chaos as hundreds of thousands of people, including refugees from war in the Middle East, arrive on the bloc's southern and eastern edges and trek to rich countries further north and west.

EU interior ministers held crisis talks, with Germany, France and the bloc's executive Commission trying to overcome opposition from eastern members to a plan to compulsorily relocate 160,000 refugees from Italy, Hungary and Greece.

A draft agreement included strong language on the need for tighter controls of the bloc's external borders, rapid screening of arrivals and deportation of those without valid asylum claims, to help assuage countries concerned that relocating asylum seekers could attract more people.