A union representing cabin crew in Germany says it's expanding a planned strike Sunday at Lufthansa to include employees of the airline's subsidiaries.

The UFO union said Friday that the move comes after Lufthansa rejected its pay demands and allegedly tried to intimidate staff.

UFO said the affected subsidiaries include the airlines SunExpress Deutschland, Lufthansa CityLine, Germanwings and Eurowings.

In addition to calling on members to walk out Sunday between 5 a.m. and 11 a.m., the union said it will also begin balloting members on the possibility of launching an indefinite strike next month.

The union urged travelers to check with airlines about possible delays or cancelations to flights Sunday.