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.
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.
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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.
"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.
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.