Verdi said on Friday that it was negotiating on behalf of 23,000 security workers in Germany.

Wage talks are to resume on Jan. 23, it said.

Fraport, the operator of Germany's busiest hub, said it expected severe disruption and asked passengers who were planning to start their journey in Frankfurt not to come to the airport during the strike, which is scheduled to last from 0100 to 1900 GMT.

It added that transfers between flights would most likely be possible but cautioned that it expected those passengers to experience delays and disruption.

On Thursday, more than 640 flights in Germany were cancelled due to security staff striking at Duesseldorf, Cologne and Stuttgart airports.

(Reporting by Arno Schuetze in Frankfurt, additional reporting by Thomas Seythal in Berlin; editing by Tassilo Hummel and Kirsten Donovan)