German Chancellor Angela Merkel said she was "shaken by the awful news," which broke as European leaders gathered in Vienna for a summit on the migrant crisis.

"This reminds us that we in Europe need to tackle the problem quickly and find solutions in the spirit of solidarity," Merkel said.

Police found the abandoned truck parked on the side of a major highway near Austria's border with Hungary.

Tote Flüchtlinge in LKW entdeckt Österreich Parndorf Neusiedl am See

Forensics experts search an abandoned truck near Austria's border with Hungary

"Today is a dark day... This tragedy affects us all deeply," Austrian Interior Minister Johanna Mikl-Leitner said, pledging to crack down on the people smugglers who arrange for migrants to travel to Europe, often in exchange for exorbitant fees.

Austrian Chancellor Werner Faymann also condemned smuggling networks: "Today, refugees lost the lives they had tried to save by escaping, but lost them in the hand of traffickers."

Tens of thousands of refugees fleeing violence in the Middle East, Asia and Africa risk their lives each year for a chance to come to Europe. Some attempt the perilous sea journey across the Mediterranean. Others take the land route through the Western Balkans. Traveling through Serbia into Hungary is especially popular with asylum seekers hoping to enter the European Union - and Thursday's deaths show it can also be dangerous.

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