Crowded aboard buses and trains, thousands more migrants flooded into Austria on Sunday as at least 13 desperate refugees drowned making the perilous trip to Europe in search of a better life.
Six children were among those who died off the coast of Turkey after the inflatable dinghy carrying them to Greece collided with a ship, Turkish media reported.
As several thousand more migrants arrived in Austria from Hungary via Croatia, Budapest abruptly decided to reopen a border crossing with Serbia whose closure on Tuesday 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 those undertaking the gruelling journey across the Balkans into western Europe, with Croatia saying more than 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 influx of arrivals and began to redirect the migrants back towards Hungary or towards Slovenia, sparking angry reactions from both countries.
Many 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.
At the Austrian town of Nickelsdorf on the Hungarian border, some 7,000 refugees and migrants were stuck waiting for onwards transport to elsewhere in Europe - some of them in a long snaking line for buses, others hoping for taxis to take them to Vienna.
"Once you get to Austria, you've arrived," said Saeed, a 23-year-old from Damascus who is hoping his odyssey will end in Germany.
"As we approach the Europe that we want, people are getting nicer and nicer."
There were more bottlenecks elsewhere along the long migrant trail up from Greece through the Balkans, as authorities in a string of countries struggle to cope with the inflow.
At Tovarnik in Croatia, on the border with Serbia, the interior ministry said nearly 4,000 people were waiting for transport towards Hungary. Buses and trains were arriving constantly, but not quickly enough to keep up with the relentless pace of arrivals.
Hungary reinforces its border
Croatian state-run broadcaster HRT reported a convoy of Hungarian military vehicles arriving at the two countries' border crossing at Beremend, and aired images of officials placing several large metal panels across a road on the Hungarian side.
Workers also arrived with fencing pillars and barbed wire in an apparent effort to reinforce Hungary's border.
The right-wing government in Budapest has already built a razor-wire barrier along much of its border with Croatia, after sealing off its frontier with Serbia in a bid to keep migrants out.
It appeared to be letting arrivals squeeze through from Croatia slowly on Sunday, with only half of the 10 waiting buses waiting at Beremend allowed through by the afternoon, Croatian national radio reported.
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.
The continent's worst migration crisis since World War II has caused a deep rift between EU members over how to distribute the arrivals.
The massive influx 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.
The foreign ministers of Poland, the Czech Republic, Slovakia and Hungary are set to discuss the crisis on Monday with their counterpart from Luxemburg, which currently holds the EU presidency.
'We lost the children'
In Sunday's accident off the Turkish coast, the dinghy that was hit was carrying at least 46 migrants to Lesbos, one of several Greek islands inundated in recent months by tens of thousands of people arriving from Syria, Afghanistan and elsewhere in the Middle East and South Asia.
A survivor whose name was given as Haseen told Greek state news agency ANA: "It was dark, we saw the ship bearing down on us. We tried to signal with flashlights and cellphones but they did not see us."
Thrown overboard, the passengers fought to keep their heads above water. "We lost the children. We could not see them in the dark," Haseen said.
More than 2,600 people have died among the nearly half a million who have braved dangerous trips across the Mediterranean to reach Europe so far this year.
The Italian navy said it carried out two operations off the Libyan coast Sunday, rescuing 146 migrants from a fishing boat in the first incident and another 339 in the second, while the Irish Niamh vessel rescued 125 people from a dinghy.
And the Libyan coastguard said it had rescued 215 migrants 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 risky sea crossing.
EU interior ministers are to meet again on Tuesday, followed by an emergency summit on Wednesday.
EU Enlargement Commissioner Johannes Hahn said the bloc was earmarking aid of "up to one billion euros" ($1.13 billion) to encourage Syrian refugees in Turkey to stay there.
Of the more than four million Syrians who have fled their country, nearly half have sought shelter in Turkey, while more than a million are now living in Lebanon and at least 600,000 in Jordan.