BERLIN, March 22 (Xinhua) -- Security staff at most of Germany's major airports went on strike again on Tuesday, leading to cancellations and delays across the country.

       The labor union Verdi called for renewed strike action after no agreement was reached in the fourth round of negotiations. The union is seeking to increase wages for around 25,000 security staff employed nationwide.

       Frankfurt airport, the largest in Germany, closed all security checks outside the transit area. It was therefore impossible to board any flights, airport operator Fraport said.

       "To seriously disrupt air traffic throughout Germany again within a very short time is completely disproportionate," said a spokesperson for Fraport, appealing to partners to negotiate an amicable solution.

       Meanwhile, at Cologne Bonn airport, security checks were completely closed for passengers. Airlines had already canceled most flights in advance, the airport said.

       Labor union Verdi is not satisfied that according to employers' latest offer, hourly wages of less than 13 euros (14.34 U.S. dollars) would only rise by 38 euro cents per hour.

       Aviation security staff in Germany are demanding a wage increase of at least one euro per hour over a twelve-month period. Negotiations are set to continue in Frankfurt on March 24. (1 euro = 1.10 U.S. dollars)