Ground workers began striking on Thursday at Frankfurt Airport, Europe's third busiest airport, which has caused more than 150 flight cancellations.

The strike took place from 3 p.m. until 10 p.m. (CET). Workers are expected to strike on Friday, too. The Financial Times reports that strikes could last until the weekend.

The strike is the result of a dispute between the GdF Union (representing air traffic control employees) and Frankfurt Airport (also known as Fraport) over pay and working conditions of roughly 200 ground workers.

Lufthansa, the flag carrier for Germany, cancelled about 100 flights during the seven-hour strike on Thursday. Without the ground crew, airlines are unable to guide aircrafts on the Tarmac and perform several other duties prior to aircraft departure.

If the strike carries into Friday, Lufthansa expects to cancel 250 flights to and from Frankfurt Airport. Ground workers are planning to strike from 8 a.m. to 10 p.m. Most long-haul services would operate mostly as normal according to a Washington Post report.

The dispute is especially unique, reports Financial Times, because German trade unions traditionally represent much larger groups. We feel we are being blackmailed, said a Frankfurt Airport spokesperson.

The union remains completely intransigent and irresponsible in its demands for high, double-digit pay increases and other adjustments for working hours and special benefits ranging between 50% and 705, Herbert Mai, Fraport AG's executive board member for labor relations, said Wednesday according to the Wall Street Journal.