Forensic police officers work next to a parked truck in which up to 50 migrants were found dead, on a motorway near Parndorf, Austria August 27, 2015. [Photo/Agencies]

VIENNA -- The tragedy in which between 20 and 50 dead illegal immigrants were found in the back of a truck in Austria on Thursday shows Europe must work together to tackle the asylum seeker crisis, German Chancellor Angela Merkel said.

"We are all shaken by the horrible news that up to 50 people lost their lives," she said at a conference for Western Balkans leaders in Vienna.

The immigrants had been found in the back of the truck parked alongside the main motorway between Vienna and Budapest, their bodies already decomposing.

The German leader urged "solidarity in the European spirit" to resolve the migration issue.

Along with Merkel and other German politicians, numerous Austrian leaders also commented on the tragedy, including Minister of Foreign Affairs Sebastian Kurz, who in a statement said he was "deeply shaken" by the incident, and also called for European cooperation in combating the crisis.

Interior Affairs Minister Johanna Mikl-Leitner has also earlier referred to a "dark day" for Austria, and expressed her view that the people smugglers responsible are "criminals."