Workers at German warehouses of U.S. online retailer Amazon.com Inc were called out on a new strike by labor union Verdi on Monday as part of a long-running dispute over pay and conditions.

Verdi said in a statement that workers at six of the nine Amazon warehouses were joining the strike, which will run until December 24 at four of the centers and for shorter periods at the others.

Germany is Amazon's second-biggest market after the United States with 10,000 warehouse staff plus more than 10,000 seasonal workers.

A Verdi spokeswoman said the union expected a similar number of workers to join the strike as previously, when hundreds of staff walked off the job at several warehouses.

But an Amazon spokeswoman said only a small minority of staff were on strike.

"There is absolutely no impact on our customer delivery promise. The vast majority of our employees are working," she said, noting that Amazon uses a network of 29 warehouses across Europe to fulfill orders.

Verdi has organized frequent strikes at Amazon since May 2013 as it seeks to force the retailer to raise pay for warehouse workers in accordance with collective bargaining agreements in Germany's mail order and retail industry.

Amazon has repeatedly rejected the union's demands, saying it regards warehouse staff as logistics workers and that they receive above-average pay by the standards of that industry.