A large number of scheduled flights were canceled at Frankfurt Airport on Tuesday morning as security staff went on strike.

Freight and passenger security workers at the airport, Germany's biggest, started their strike at 2 a.m. (1 a.m. GMT), a spokesperson for the Verdi union said in the morning. Emergency transit workers were exempted from the strike.

The strike action is part of a contract dispute between Verdi and the German federal aviation security association BDLS.

The union is negotiating with the employers' association for a new contract for around 25,000 security workers nationwide. Three rounds of talks have already yielded no results.

Hamburg, Stuttgart and Karlsruhe/Baden-Baden are also expected to see staff strike on Tuesday. At Germany's second-largest airport in Munich, staff have been on strike since Monday afternoon.

Frankfurt Airport's website indicated many departing flights had been canceled on Tuesday morning. The airport operator Fraport had previously asked travelers who wanted to board in Frankfurt not to travel to the airport.

An emergency service was set up for passengers transferring through Frankfurt, a Verdi spokesperson said.

Further negotiations are due to take place on Wednesday and Thursday. The union's demands include an increase in hourly wages of at least 1 euro ($1.10) per hour.