Workers at Amazon, the world's biggest online retailer, began two-day strikes Monday at four of the Internet giant's German distribution centers in a long-running dispute over wages.

Germany's Verdi union said employees at Amazon's largest European logistics center in Bad Hersfeld would be the first to walk off the job, followed by workers in Leipzig, Graben and Rheinberg. The strike will last until Tuesday night.

"Amazon continues to deny workers their right to a collective bargaining agreement, which is guaranteed by law in Germany," said union leader Stefanie Nutzenberger.

For more than a year, Amazon and the Verdi union have been locked in an acrimonious battle over higher pay. Verdi wants Amazon to categorize its employees as retail - and not logistics - workers, which would qualify them for increased wages.

Amazon employs some 9,000 people at nine locations across Germany.

cjc/hg (AP, Reuters, AFP, dpa)