Hundreds of refugees on Monday tried to break through a border fence into Macedonia from Greece, where more than 7,000 people are stranded, as anger mounted over barriers to entry imposed on migrants.

In a sign of widening divisions within the European Union, German Chancellor Angela Merkel meanwhile lashed out at Austria and Balkan states on the migrant route for abandoning debt-laden Athens to refugee chaos.

And Macedonian President Gjorge Ivanov warned that once Austria reaches its limit on migrant entries this year, the Balkan refugee route will have to close.

On the frontier, Macedonian police fired tear gas as some 300 migrants forced their way through a Greek police cordon and raced towards a railway track between the two countries.

"Open the borders!" they shouted as a group of men used a metal signpost to bring down a section of barbed wire fencing, prompting police to fire volleys of tear gas and prevent them from crossing.

At least 30 people, many of them children, requested first aid in the stampede that ensued, the charity Doctors Without Borders (MSF) said.

Skopje said one of its policemen had been hurt and required hospitalisation.

The protest occurred several hours after Macedonia allowed just 300 Syrians and Iraqis to cross.

With Austria and Balkan states capping the numbers of migrants entering their soil, there has been a swift build-up along the Greece-Macedonia border with Athens warning that the number of people "trapped" could reach up to 70,000 by next month.

The UN's rights chief criticised a "rising roar of xenophobia" towards migrants.

"To keep building higher walls against the flight of these desperate people is an act of cruelty and a delusion," Zeid Ra'ad Al Hussein said.

Amnesty International criticised the situation at Idomeni, branding it "the result of a shameful spate of discriminatory border closures."

Authorities in the Netherlands meanwhile said they had identified 30 war crimes suspects including 10 from Syria among asylum applicants in 2015.

As the bottleneck showed little sign of easing, Merkel slammed the restrictions and pointed the finger at Austria, whose introduction of restrictions on February 19 triggered a domino effect in the Balkans.

"We can't just abandon this country," she said, referring to Greece.

The spate of border closures was sparked by Austria's announcement it would accept no more than 80 asylum claims per day and cap the numbers of those seeking to cross its territory.

 'Not my Europe'

 "Because Austria decided on a limit of 80 per day, and not one more, we have reached today's situation," Merkel said in a TV interview with public broadcaster ARD Sunday.

"When one insists on his border, the other suffers. That's not my Europe."

Macedonia's Ivanov said in an interview with German news website Spiegel Online that once Austria's limit of 37,500 entries this year is reached, the Balkan route will close.

"When Austria reaches its limit, it will happen," he said.

Asked when that might occur, he replied "perhaps right at this moment".

The European Commission said it was working on contingency plans to help Greece and other western Balkan countries cope.

On the ground, thousands continued to mass on the Greek-Macedonian border.

The build-up at Idomeni camp, which can accomodate up to 1,500 people but is sheltering more than 7,000, began in earnest last week after Macedonia began refusing entry to Afghans and imposed stricter controls on Syrians and Iraqis.

EU members Slovenia and Croatia quickly followed suit along with Serbia, with all four states imposing a daily limit of 580 migrants.

"Do you seriously believe that all the euro states that last year fought all the way to keep Greece in the eurozone -- and we were the strictest -- can one year later allow Greece to, in a way, plunge into chaos?" Merkel said.

 'Absurd' criticism

Austria quickly hit back at criticism, describing it as "absurd".

"We don't have to take criticism from anyone on any side," Interior Minister Johanna Mikl-Leitner told the Austria Press Agency.

"Apparently for some, the European solution (to the crisis) is for all (migrants) to mass in Austria," she said. 

In Athens, ministers held an emergency meeting to determine how to handle the looming deadlock.

With facilities in the capital nearing breaking point and hundreds of people continuing to arrive at the port of Piraeus, authorities hastily opened two indoor facilities -- one of them at a baseball stadium disused since the Athens 2004 Olympics -- to shelter another 2,000 migrants.

In France meanwhile, authorities began bulldozing half of the "Jungle" migrant camp in the port city of Calais where thousands of migrants hope to sneak aboard lorries and ferries to Britain.

Belgium has turned back 619 migrants at the French border since reinstating border checks, a police spokesman told AFP.