BERLIN - Amazon staff at German warehouses extended a strike on Wednesday in a long-running dispute over better pay and conditions, but the online retailer promised it would be able to deliver Christmas orders on time.

Labour union Verdi launched a three-day strike on Monday at five of Amazon's nine distribution centers in Germany, adding a sixth warehouse from Tuesday, with more than 2,600 workers involved in the industrial action on Wednesday.

Verdi said in a statement that workers at four of those centers had decided to continue their strike until Saturday and employees at the Graben warehouse would strike until Dec. 24.

An Amazon spokeswoman said the company had not seen any delays so far and had actually extended the deadline for customers to get gifts in time for Dec. 24 by normal delivery until midday on Dec. 22 from midnight on Dec. 21.

"We keep our customer promises," she said.

Amazon said only a minority of staff had joined the strike, with around 19,000 working normally.

The U.S. company employs almost 10,000 regular staff at its warehouses in Germany, its second-biggest market behind the United States, as well as more than 10,000 seasonal workers. It can also draw on 19 other warehouses across Europe.

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 across 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.

The Verdi union said more than 1,000 Amazon workers had joined a rally in Koblenz on Wednesday, including employees from three other warehouses.

"Your courage and your determination will bring Amazon to the negotiating table. You are an example to the whole country," Verdi board member Stefanie Nutzenberger told the strikers.