SERBIAN-HUNGARIAN BORDER: Hundreds of migrants spent the night in the open on Serbia’s northern border with Hungary, their passage to western Europe stalled on Tuesday by a Hungarian crackdown to confront the continent’s worst refugee crisis in two decades.

Aid workers brought tents, food and water for about 500 migrants, many of them Syrian refugees, denied entry overnight under a strict new border regime that EU member Hungary warned many see many asylum seekers rejected and expelled back to Serbia.

Reuters reporters saw dozens more pass through gaps in an unfinished fence that Hungary is building the length of the EU’s external border with Serbia, squirreling under coils of razor wire.

Some made it, while others were stopped before they could try and directed to the official border crossing by police and soldiers. Some, tired and cold, sat down in fields short of the 3.5-metre high metal fence.

A major flashpoint in Europe's worst refugee crisis since the Yugoslav wars of the 1990s, Hungary has vowed to clamp down on the flow through the Balkan peninsula to the richer countries of northern and western Europe.

As ministers of the EU’s 28 member states argued over how to share out responsibility for some of the hundreds of thousands who have sought asylum in Europe this year, Hungary under right-wing Prime Minister Viktor Orban moved on Monday to close down their main land route into the bloc along railway tracks from Serbia.

"We will start a new era," government spokesman Zoltan Kovacs said shortly after midnight on the border. "We will stop the inflow of illegal migrants over our green borders."

Police have recorded over 190,000 entering Hungary this year, including a record for one day of more than 9,000 on Monday.

Their window of opportunity is rapidly shrinking as Hungary puts the finishing touches to the fence the length of its 175-km (108 mile) southern border.

On Monday, a cargo wagon, one end covered in razor wire, was shunted into position to close the main informal crossing point for migrants. Helmeted police and soldiers stood guard and a helicopter circled overhead.

“We’re on the street now,” said Mouz, a 22-year-old Syrian, who slept on the border. Asked if he might consider another route, possibly through Serbia’s western neighbour Croatia, he replied: “I don’t know. I’m from Syria. I cannot go back.”

The government says refugees will still be able to claim asylum at two official border crossings into Hungary, but if they have entered from Serbia – and not already sought asylum there – they face automatic expulsion within eight days after Budapest in July declared its impoverished southern neighbour 'safe' for refugees.

The United Nations refugee agency, UNHCR, says it disputes the designation of Serbia, not an EU member, as a so-called "safe third country", which would imply refugees have a fair chance of being granted asylum and will receive all the necessary protections and support.

Rights groups say Serbia meets none of the criteria and is still finding homes for thousands of its own refugees from the collapse of Yugoslavia in the 1990s, the last time Europe confronted displacement of people on such a scale.

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Orban, one of Europe's most vociferous opponents of mass immigration, said he expected a high rate of deportations.

"In such a case, if someone is a refugee, we will ask them whether they have submitted an asylum request in Serbia. If they had not done so, given that Serbia is a safe country, they will be rejected," he was quoted as telling private broadcaster TV2.

An official of Orban's Fidesz party said authorities would rule on such asylum requests within eight days.

Orban has vowed zero tolerance on the EU's external border, framing the crisis as a battle for Europe's prosperity, identity and "Christian values".

The influx into Europe, by boat from North Africa across the Mediterranean or across Turkey and up the Balkan peninsula, has triggered discord and recrimination in the 28-nation EU, feeding anti-immigration sentiment.

On Monday, two decades of frontier-free travel across Europe unravelled as Austria and Slovakia followed Germany in re-establishing border controls to cope with the influx. Austria said it would dispatch armed forces to guard its eastern frontier with Hungary.

EU ministers meeting on Monday failed to break a deadlock over sharing out responsibility for some of the hundreds of thousands who have sought asylum in Europe this year.

Migrants were able to enter Hungary until midnight on Monday, in small groups queuing at an official pedestrian border crossing, but there was deep uncertainty over a possible bottleneck on the Serbian side on Tuesday as thousands continued to stream through the Balkans from Greece, having arrived by boat and dinghy from Turkey.

Aleksandar Vulin, the Serbian government minister in charge of policy on migrants, said Serbia would not accept anyone being returned to Serbia having already entered Hungarian territory.

"That's no longer our responsibility," he told the Tanjug state news agency. "They are on Hungarian territory and I expect the Hungarian state to behave accordingly towards them."