Employees of Amazon's German branch protest on July 18. Photo: IC

German labor union Verdi is preparing more strikes to step up pressure on Amazon, the world's biggest Internet retailer, in a dispute over pay and conditions, a newspaper reported.

"Further strikes are to be expected," Welt am Sonntag quoted Verdi board member Stefanie Nutzenberger as saying in an interview published on Sunday.

The union has organized several short strikes this year in a bid to force Amazon to ­accept a collective agreement on employment conditions similar to deals for the mail order and retail sector, which are more generous than for the logistics sector.

Amazon regards staff in Bad Hersfeld and Leipzig as logistics workers and says they receive above-average pay by the standards of that industry.

Nutzenberger said Verdi, which had already warned its members could strike during the busy Christmas holiday season, could expand industrial action beyond Leipzig and Bad Hersfeld, which have been the focus of walkouts so far.

"And we will concentrate on days that will be especially disruptive to business," she said.

Amazon employs around 9,000 people in ­Germany, its largest market outside the US. Sales there grew almost 21 percent in 2012 to $8.7 billion, representing one-third of its overseas total.

"So far the strike action by Verdi has had no impact on shipments to our customers," Amazon spokesman Stefan Rupp told Reuters in an e-mailed statement, affirming the company sees no benefit for its workers in the kind of collective agreement Verdi demands.