FRANKFURT (Reuters) - Union Verdi is preparing more strikes to step up pressure on Amazon in Germany, its biggest market outside the United States, in a dispute over pay and conditions, German media reported. Workers at Amazon centres in Bad Hersfeld and Leipzig will walk out for at least one day on Monday, Heiner Reimann of Verdi told daily Stuttgarter Nachrichten. The union has organised several short strikes this year in a bid to force the world's biggest Internet retailer 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. Verdi board member Stefanie Nutzenberger told weekly Welt am Sonntag that the union could extend industrial action beyond those two centres during the busy Christmas holiday season. "And we will concentrate on days that will be especially disruptive to business," she was quoted as saying in an article published on Sunday. Amazon employs around 9,000 people in Germany. Sales there grew almost 21 percent in 2012 to $8.7 billion, representing a 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 said in an e-mailed statement. He affirmed the company's stance that it sees no benefit for its workers in the kind of collective agreement Verdi demands. (Reporting by Maria Sheahan; editing by Andrew Roche and William Hardy)