A spokesman for Vereinigung Cockpit (VC), the union that represents the pilots at Germany's flag carrier airline, said on Tuesday that they could soon call more strikes in addition to the walkout affecting long-haul routes, which began at 0600 UTC, and the strike of short- and medium-haul flights, set to start on Wednesday.

"We cannot rule out further strikes this week," VC spokesman Markus Wahl told reporters at Frankfurt Airport, Lufthansa's main hub on Tuesday. "Strikes are possible in the following weeks as well," he added.

Tuesday's strike - which is to run until just before midnight local time (2159 UTC) -has forced Lufthansa to cancel a total of 84 flights, affecting around 20,000 passengers. That's almost half of the airline's regularly scheduled long-haul flights. Ninety long-haul flights from Frankfurt, Munich and Düsseldorf were expected to go ahead as scheduled.

Despite the disruption the situation appeared to be calm at the affected airports, with few passengers turning up for cancelled flights as the airline had earlier warned them ahead of time, using text messaging and email. Extra service staff was also on hand to assist the few who did turn up for cancelled flights.

Both Lufthansa and the airline's passengers have grown accustomed to such disruptions, as this is the 13th time that the pilots have walked off the job within the space of a year and a half - and there is no indication that Tuesday's or even Wednesday's will be the last.

"We are determined," an unnamed Lufthansa spokeswoman told the Reuters news agency, adding that there was no obvious solution to the labor conflict. "Cockpit is on the wrong path," she added.

Despite several rounds of negotiations, the union and management have been unable to resolve their long-running dispute over early retirement provisions. The air carrier wants to cut costs by doing away with so-called 'transition contracts' handed to pilots who stop flying before the legal age of retirement. These allow pilots to retire as early as age 55 while receiving up to 60 percent of their salary until they reach retirement age.

Added to this dispute is Lufthansa management's plan to expand the operations of its low price unit Eurowings. It is incorporated in Austria, where German labor agreements reached in collective bargaining do not apply.

Tuesday's walkout was the first since just before the crash of a jet operated by Lufthansa subsidiary Germanwings in March.

pfd/kms (dpa, Reuters, AFP)