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BERLIN (AP) — A German union has called on Lufthansa ground staff to walk out on a one-day strike Wednesday in a dispute over pay.

The ver.di service workers’ union said Monday that the call applies to all Lufthansa locations in Germany. It comes amid negotiations on pay for about 20,000 employees of logistical, technical and cargo subsidiaries of the airline.

The strike was set to start at 3:45 a.m. (0145 GMT) Wednesday and end at 6 a.m. (0400 GMT) on Thursday.

The union aims to raise the pressure on Lufthansa ahead of the next round of negotiations on Aug. 3 and 4.

Such “warning strikes” are a common tactic in German labor negotiations and typically last from several hours to a day or two. This walkout comes at a time when airports in Germany and elsewhere already are seeing disruption and long lines for security checks.

Ver.di is calling for a 9.5% pay increase this year and says an offer by Lufthansa earlier this month, which would involve a deal for an 18-month period, falls far short of its demands.

Lufthansa human resources chief Michael Niggemann said in a statement that ver.di’s strike call “is all the more incomprehensible given that the employer side has offered high and socially balanced pay increases” despite a “tense” economic situation for the company and an uncertain economic outlook.