Workers at German warehouses of online retailer Amazon.com took strike action again on Monday as labor union Verdi pressed its demands in a long-running dispute over pay and conditions.

Verdi said in a statement it had called out workers to strike at distribution centers in Bad Hersfeld, Leipzig, Graben and Rheinberg.

Verdi had in June staged walkouts at three of those sites.

The U.S. company employs a total of 9,000 warehouse staff at nine distribution centers in Germany, its second-biggest market behind the United States, plus 14,000 seasonal workers.

Verdi wants Amazon to raise pay for workers at its distribution centers in accordance with collective bargaining agreements across the mail order and retail industry in Germany and has organized several stoppages over the past year.

Amazon, however, has rejected the demand, arguing that it regards warehouse staff as logistics workers and says they receive above-average pay by the standards of that industry.