Nearly all flights at Germany's Cologne-Bonn airport and the majority at nearby Duesseldorf have been cancelled or diverted today (27 February) due to strikes.
The walkouts are also affecting local transportation, day-care facilities and local administration in Germany's most populous region.
At Cologne-Bonn airport, the strikes commenced on Sunday evening. On Monday, all but two of the day's 136 planned flights are expected to go ahead, German news agency dpa reports.
In Dusseldorf, only 89 of the planned 330 flights are expected to take place as scheduled, with most of the rest being cancelled.
The strikes at both airports are expected to end overnight between Monday and Tuesday.
Eurowings airline, a subsidiary of Lufthansa, says it "expects that flight operations will be back to normal as early as Tuesday".
Why are airport workers striking in Germany?
The one-day 'warning strike' comes amid difficult pay talks for employees of Germany's federal and municipal governments and for airport security staff.
Walkouts planned Monday also were set to affect buses and trams in parts of the western state of North Rhine-Westphalia, as well as day-care centres for children and other services.
Unions are seeking a 10.5 per cent pay raise. Employers so far have offered an increase totalling 5 per cent in two stages and one-time payments of €2,500 per employee - which unions have rejected as insufficient.
On Friday 17 February, warning strikes organised by trade union Ver.di NRW led to thousands of cancelled flights at seven German airports. Lufthansa was forced to cancel all flights from Frankfurt and Munich just days after an IT failure caused major disruption for the airline.
Various strikes are planned across Europe in the coming months, including in Spain, France and the UK.