"We will continue to strike, also next year," Verdi representative Joerg Lauenroth-Mago said. "But I won't say when and where exactly that will happen."

Hundreds of workers at Amazon's logistic centers in Bad Hersfeld and Leipzig have been on strike since Monday, in actions due to end on Saturday, December 21, threatening to disrupt shipments in the Christmas shopping season. Some staff at the company's Graben site have also stopped work.

Amazon said that, on average, only 900 staff joined the strike and that pre-Christmas deliveries had not been affected. The company employs a total of 9,000 warehouse staff in Germany plus 14,000 seasonal workers at nine distribution centers.

"The overwhelming majority of our workers are fully committed, so all packages are making it in time for the holiday," Amazon's Steven Harman, who is responsible for the German logistics centers, said in a statement.

Union Verdi has organized short strikes this year to try to force Amazon to accept collective bargaining agreements, which are used elsewhere in the mail order and retail industry, as benchmarks for pay at Amazon's German distribution centers.

However, Amazon has maintained that it regards staff at its centers in the cities of Bad Hersfeld and Leipzig as logistics workers and that they receive above-average pay by the standards of that industry.

Verdi members called their latest stoppages this week in the busy days before Christmas, when it would hurt the online retailer the most.

Amazon said orders in Germany peaked on December 15, with 4.6 million products ordered in one day - 53 per second. It said that was 15 percent above order levels a year earlier.