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.

"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.

EU foreign policy chief Federica Mogherini told participants at the conference in Vienna she hoped the tragedy would push member states to "take decisions and responsibility."

Investigations underway

Austrian police have launched an investigation to establish how the migrants in the truck perished, and who may be responsible for their deaths.

Authorities discovered the vehicle at around midday on Thursday, and soon realized something was wrong when they noticed blood dripping from the cargo area and the smell of dead bodies.

The truck was later towed to an air-conditioned location near the border with Hungary, where workers in white protective suits could be seen wheeling body bags into a building.

Forensics experts worked through the night to find out what they could before the bodies are transferred to Vienna on Friday for further examination.

Senior police official Hans Peter Doskozil said he'd received information to suggest the truck had crossed the border from Hungary into Austria overnight before it was abandoned. He also said it was likely the victims were already dead before the truck entered the country.

nm/lw (Reuters, AP, AFP)