BERLIN (AFP) - Germany's giant services sector union Verdi is turning up the pressure on online retail giant Amazon over the busy Christmas holiday period with fresh strikes over pay.

Employees at two Amazon sites in Germany, Leipzig and Bad Hersfeld, staged walkouts early on Monday, calling for Amazon to align its pay with those usual for the retail and mail-order sectors.

A one-off bonus of 400 euros (S$678) granted to mail order staff and 600 euros to other staff was insufficient, Verdi argued.

"It's up to Amazon whether there will be further strikes in the Christmas period. As soon as Amazon agrees to talk, we'll return to the negotiating table," said Verdi official Mechthild Middecke.

But in an interview with the daily Die Welt, Amazon's global logistics chief Dave Clark said he did not have much time for Verdi, preferring instead "direct relations with our employees, via works councils and employee forums". Verdi has been trying for months to get Amazon to bring the pay of its 9,000 workers in Germany in line with wages in the distribution sector.

Amazon refuses, arguing that its distribution centres are logistic sites and that it pays its staff accordingly. Wages in the logistics sector in Germany are lower than in the distribution sector.

Amazon's German headquarters are in Munich. But it also operates eight logistics centres and two customer service centres.

Some 3,300 people work on two sites in Bad Hersfeld and 2,000 in Leipzig in the east of Germany.

Amazon's employment practices were the subject of a critical documentary broadcast on German public television at the beginning of the year.

Last month, Amazon said it plans to open two distribution centres in the Czech Republic by the end of 2014.

The centres are a response to increased demand across Europe, Amazon said in a statement, adding that it chose the Czech Republic for its prime location in the heart of Europe.