BERLIN (AP) — Air travel was disrupted across Germany on Tuesday as security personnel at several airports in the country staged walkouts to demand higher wages.

Numerous flights were canceled at Frankfurt Airport due to the strike organized by trade union ver.di, German news agency dpa reported. Starting at 2 a.m., employees of the cargo and passenger controls at Germany’s largest airport stopped working, a ver.di spokesperson said in the morning. Only passengers with layovers were able to go through security checks in Frankfurt. Airport operator Fraport had called on all travelers hoping to board in Frankfurt not to travel to the airport.

According to Fraport, 130 of 818 flights planned for Tuesday were canceled. Originally, about 71,000 passengers had been expected at the airport that day.

Employees at Hamburg, Stuttgart and Karlsruhe/Baden-Baden airports also walked out of their jobs. In Munich, Germany’s second-largest airport, a walkout has been underway since Monday afternoon. Other airports including Berlin, Duesseldorf and Hannover canceled dozens of flights on Monday due to one-day strikes there.

The walkouts are part of a wage dispute between ver.di and the Federal Association of Aviation Security Companies. The union is negotiating with the employers’ association for a new agreement for about 25,000 security staff nationwide. Ver.di is demanding an increase in hourly wages of at least one euro, among other things.

Three rounds of negotiations have so far failed to produce a result. The sides plan to meet in Berlin later this week for further negotiations.