ZAGREB (AFP) - Road and rail routes to northern Europe from the Balkans were closing to migrants on Friday (Sept 18), after a string of countries shuttered their borders to the relentless human wave.

Almost a quarter of a million people applied for asylum in the European Union between April and June, new figures released in Brussels showed.

The massive influx, challenging the EU's humanitarian reputation and its vaunted policy of border mobility, has triggered an emergency summit of the 28-nation bloc on Wednesday.

After initially vowing to let the refugees through, Croatia on Thursday said it was overwhelmed by the influx, and closed seven of its eight crossing points with Serbia.

"Up to now we have registered 13,000 migrants on Croatian territory," Interior Minister Ranko Ostojic told N1 television station on Friday. "This figure means that our capacities to take in more are saturated."

Migrants tried to continue their northward trek by train through Croatia's neighbour, EU member Slovenia, which then announced it was suspending all Croatian rail links until late Friday.

Scenes of chaos and despair at the Croatian-Serbian border brought home the human scale of the problem.

Thousands were camped in the fields around the rural town of Tovarnik, whose little train station has become the latest flashpoint in a months-long drama.

- Landmine warning -

A train there had been waiting to leave since 4am (10am Singapore), Peter Bouckaert, Human Rights Watch emergencies director, told an AFP reporter.

"It's pretty dramatic. People are getting very angry. If the train doesn't leave they're going to start fighting," he warned.

"It's complete chaos. There are still thousands waiting after spending a rough night with little food or shelter. This is a one-street town which is completely overwhelmed."

Bouckaert added: "If people avoid the official crossing, they could end up in landmine fields."

In Harmica, a small village on the Croatian side of the border with Slovenia, a couple in their 30s with a four-year-old boy and a nine-year-old girl gave details of their gruelling trip through Europe to escape fighting in Syria.

"We've been travelling for six days, first across the sea and then through Greece, Macedonia and Serbia," the young woman said. "We left Syria because of the war, we want to get to Germany."

Hungary came under renewed criticism on Thursday, a day after riot police on its Serbian border fired tear gas and water cannon at migrants enraged at being blocked from entering the country.

UN rights chief Zeid Ra'ad Al Hussein said Hungarian government policy was apparently being guided by "xenophobic and anti-Muslim views", while Serbian Prime Minister Aleksandar Vucic said: "This torture and non-European behaviour must stop."

Unabashed, Hungary's hardline conservative premier, Viktor Orban, announced Friday that hundreds of troops had begun laying a razor-wire barrier along the country's border with Croatia.

The barrier will add to a fence that Hungary has set down alongside its border with Serbia.

"Forces are being redeployed, 600 soldiers are already at the scene, 500 will arrive there in the course of the day, and several hundred at the weekend. Two hundred police have been assigned to the Croatian border," Orban said.

"There will be no sandhill or molehill to hide behind, we will defend our borders."

- EU's deep split -

The EU received 213,000 asylum applications from April to June this year, up 85 per cent from the same period in 2014, with over one third coming from Syria or Afghanistan, official figures showed Friday.

Germany had the highest number of new applicants, with more than a third of the total for the EU, while Hungary was the country that received the most asylum applications relative to its population size, the EU's Eurostat statistics agency said.

The influx - the greatest migratory flow in Europe since the end of World War II - has dug a deep rift between western and eastern EU members, with Orban leading the hardline group.

EU nations are battling over how, or whether, to share out the refugee flow.

Germany on Friday warned that it could invoke EU's majority voting system to force reluctant states to accept quotas of migrants.

Another worry is that the proliferation of frontier controls within the EU could wreck the Schengen accord, a pillar of the European project which allows borderless travel between member states.

Germany, Austria and Slovakia have all reimposed identity checks on parts of their borders, and Poland and the Netherlands are considering whether to follow suit.

And under a draft law seen by AFP, German Interior Minister Thomas de Maiziere is seeking to toughen asylum laws by sending migrants back to the first EU country they reached and by reducing benefits.

If passed, the law would represent a major reversal on Germany's easing of asylum laws for Syrians, a factor contributing to the influx that has left Germany bracing for up to a million asylum seekers this year.

EU President Donald Tusk has called for an emergency summit next Wednesday.

French President Francois Hollande has said he would use the summit to ask Turkey to keep Syrian refugees on its soil until the war ends.

German Foreign Minister Frank-Walter Steinmeier was due in the Turkish capital Ankara Friday for talks on the migrant crisis.