BEREMEND, Hungary - Crowded aboard buses and trains, thousands more migrants flooded into Austria on Sunday from countries unable or unwilling to cope with a desperate human tide escaping war and poverty for a better life in western Europe.

And new tragedy struck for those fleeing by sea when at least 13 migrants including four children died off the coast of Turkey after the inflatable dinghy carrying them to Greece collided with a ferry, Turkish media reported.

As several thousand more migrants arrived Sunday in Austria from Hungary via Croatia, Budapest abruptly decided to reopen a border crossing with Serbia whose closure on Monday had sparked a surge of migrants into Croatia.

The Horgos-Roszke 1 crossing is on the highway that before the migrant crisis engulfing Europe was the main route linking Belgrade and Budapest.

The closure added distance and uncertainty for thousands undertaking the gruelling journey across the Balkans into western Europe, with Croatia saying 25,000 had entered its territory in the past four days.

Within days of the border closure, Croatia said it could not cope with the flow and began to redirect the migrants back toward Hungary or toward Slovenia.

Croatia, Hungary and Slovenia are all EU members, but only the latter two belong to the passport-free Schengen zone.

The bulk of the migrants are fleeing the war in Syria, with the European Union receiving almost a quarter of a million asylum requests from April through June. Germany alone expects up to a million asylum seekers this year.

On a train stopped at the Hungarian border town of Gyekenyes, across from northeastern Croatia, Ali Al-Mahmody waited with his wife and seven-month-old baby. "All I want is to see my little baby boy grow up," the 59-year-old from Baghdad, who was imprisoned under Saddam Hussein, told AFP.

The family left Iraq about a month ago, and craves sleep, he said. "We slept in forests in Macedonia. We can tolerate the hunger, but we just want to sleep."

The continent's biggest migratory flow since the end of World War II has caused a deep rift between western and eastern EU members over how to distribute the migrants.

The crisis has raised questions over the fate of the Schengen agreement allowing borderless travel across most countries within the 28-nation bloc, with several of them imposing border controls. Hungary's right-wing government in particular has faced international criticism over violent clashes with migrants and a hastily erected fence along its border with Serbia.

The Hungarian authorities began transporting thousands of migrants straight to the border with Austria on Friday in an apparent bid to move them through and out of their territory as quickly as possible. But despite heated rhetoric between Budapest and Zagreb, authorities on both sides are showing high levels of cooperation on the ground.

AFP correspondents reported that on Saturday and into the night, around 15 Croatian buses brought hundreds of migrants to Baranjsko Petrovo Selo, who crossed on foot to Beremend, in Hungary, where Hungarian buses were waiting for them.

And the Libyan coastguard said it rescued 215 migrants Sunday from two boats in the Mediterranean including more than 50 women. Many migrants have turned to Turkey's land borders with Greece and Bulgaria to avoid the sea voyage. Altogether, Greece has seen more than 300,000 migrants enter the country this year, most of them passing through to other European countries.

Meanwhile, Afghan President Ashraf Ghani on Saturday vowed to boost his country's fragile economy in a bid to retain some of the tens of thousands of his citizens seeking to migrate to industrialised countries in Europe and elsewhere. "I am aware of the bad situation of Afghans in Greece, the Balkans or those seeking refuge in Australia or other countries," Ghani told Tolo News television.

After Syrians, Afghans make up the second biggest national contingent of the huge migrant influx sweeping through Europe, with more than 50,000 making the trip since the start of the year, according to the UN's refugee agency.

The bloody ongoing conflict stemming from the ousting of the Taliban regime in 2001, corruption and the lack of economic prospects are the main factors driving Afghans to leave home.

Ghani insisted there were reasons for "hope here", pointing to agricultural projects and other initiatives to boost the economy, including the construction of the ambitious 1,800-kilometre-long (1,110 miles) TAPI gas pipeline that will eventually connect Turkmenistan to India, Pakistan and Afghanistan.

"This will all help create jobs," Ghani said.

After a decade of growth grazing the double digits, the Afghan economy has been contracting since 2013, hampered by concerns over the election that ultimately brought Ghani to power, and the withdrawal of the bulk of NATO forces at the end of 2014.