A strike by security officers over pay has disrupted travel for thousands of people in two of Germany’s busiest airports, according to an article from Reuters.

Some 400 workers walked out of Dusseldorf airport in western Germany, resulting in 183 flights being postponed until Friday, according to the airport and Verdi – an airports and service workers trade union.

At the northern airport of Hamburg, 103 flights were canceled, which affected more than 17,000 passengers, Reuters reports. Verdi called on the private sector security workers to strike from the start of the Thursday morning shift until midnight.

Verdi reports that the union was trying to force employers to negotiate, warning that further strikes could follow if this is not achieved.

The union is calling for wage rises of between 2.50 and 3.64 Euros per hour for 34,000 security workers for the state of North Rhine-Westphalia and the Dusseldorf airport. In Hamburg, the union calls for a wage hike of 2.70 Euros for around 600 security workers, Reuters says.

Works councils estimate that more than 70 percent of those employees work in the lowest wage group, earning 8.23 Euros gross ($11.06) per hour.